The idea behind this argument is that humanity has invented thousands of gods throughout history. The atheist and the Christian both reject these many gods, so rejecting gods can’t be something the Christian would object to. Why then complain when the atheist rejects that one final god? (Part 1 here.)
Let’s wrap up by exploring the last few arguments.
13. The Mere Christianity argument
When I first encountered C. S. Lewis’s Mere Christianity, I found the title to be confusing. Was he dismissing or denigrating Christianity? No, by “mere” he meant those traits of Christianity that were common among the many denominations. One source sees this kind of commonality with religions as well.

As a Christian I do believe that most other religions have many things right. I am not a Hindu, but the Hindus and I agree that there is more to the world than the material, that humans have souls, and that there is objective right and wrong. There are many aspects of Brahmin that I recognize as aspects of my own God. I simply believe that Hindus are wrong on most of the details.

The author goes on to make a similar comparison with Norse religion, noting that both he and the Norseman agree on the supernatural, life after death, and that we will be judged based on our work on earth.
It’s true that many societies share a few core supernatural beliefs, but they may also share racism and sexism (h/t Kaveh Mousavi). They may share beliefs in superstitions and astrology. Does, “Yeah, but look at all the societies that shared this thinking!” still sound like a good way to discover the truth?
And you’re making too much from this observation since you and your fellow believers can’t even agree on the names the god(s), the number of god(s), and how to placate them. Those are some substantial details to disagree on.
Can you be a Christian and declare that all roads lead to God? If not, then this all-encompassing Kumbaya thinking is just a smoke screen, and we’re back to the Christians and atheists agreeing on 99+% of the gods that don’t exist.
13b. God as unicorn.
Here’s a variation on that argument. One source imagines that he believes in unicorns. His belief is strengthened when he learns of another person who believes in them as well … though this person calls them something else and describes them completely differently. And then there’s another believer, though he also has a different name and description.

Hearing all this different accounts might make me doubt my own conception of the unicorn: but the last thing it would do is make me doubt that a unicorn exists. Instead my faith would be strengthened by that fact that all these other people did see something.

But unicorns don’t exist! Why would you use an example that would lead every thoughtful person to reject this supernatural belief?

There is a magical creature out there; it is only my own conception of it that is in doubt. In just the same way pointing out that mankind has believed in thousands of other gods and worshipped in other ways may be a decent argument against my own conception of God, but it is a terrible argument to try and make me believe that there are no gods at all. Indeed it only strengthens my faith in the supernatural.

You look around and see thousands of gods that societies have invented and that strengthens your belief in the supernatural?
Are there any gods in that pile that actually exist? Maybe, but you’ve got an uphill climb to give us good reason to believe so. The study of religion shows that mankind has a strong need to invent the supernatural, and this should only undercut your belief in it.
14. We both have gods. “God” is just the thing central to one’s life—money in your case.

Functionally-speaking, everybody has a “god”, even if they don’t have a “religion.” You have something that you’ve placed at the center of your life that gives it direction, meaning, purpose, and value. You devote your time, energy, love, and affection to this thing as if it were the most central thing in the universe. That, in the monotheistic traditions, is what is called an idol.… The point isn’t whether or not you will worship a god. The point is “which god will you worship?” (Source)

Uh oh—I think it’s tough-love time.

If you match Jesus up with the most common American god, Money, Jesus wins. Jesus is totally better than money.

And yet we know that money exists. Money 1, Jesus 0.

Money never satisfies.

While I agree that successful people can seek wealth beyond the point where it is helpful, money actually solves a lot of problems. One influential U.S. study said that money can buy happiness—happiness correlates with income up to $75,000 per year. While greed isn’t pretty, neither is poverty.

[Money] never delivers what it promises.

Sounds like Jesus! Only by making him unfalsifiable can Christians put him in the ring with a competitor that actually exists.

Jesus, on the other hand, well, he’s not going anywhere. … I could go on [for] hours, but you kinda get the point. Jesus > Money. Name anything else, even really good things, (Jesus > relationships, Jesus > your personal freedom, Jesus > sex, Jesus > power, Jesus > fame, Jesus > stuff, Jesus > a career, Jesus > status, Jesus > being a rockstar, etc.), and Jesus wins every time.

Jesus goes where you tell him to go because he’s just pretend. If you say he’s not going anywhere, then I’m sure he’s not. He’s just a meme in your mind and you’ve made him immune to evidence.
Money, relationships, career, and so on actually exist. Look at your list and notice that Jesus is the odd man out.
15. A final critique.
There are more arguments on the internet, but let’s round it out to 15.

There are problems with this reasoning. The first is that it begs the question against Christianity by assuming that there are no good reasons to be a theist (i.e. if you examined Christianity, you’d reject it too). (Source)

That’s not the way I see the argument. It makes no declaration that Christianity is false; it is just trying to provoke thought. What does it say that our positions are so similar with respect to the other religions? Sure, Christianity could be, against all odds, the needle in the haystack, but understand how much evidence you’ll need to convince us (or yourself).

There have been many who have examined Christianity and found it to be epistemologically robust; so the reasoning of the atheist is question begging.

I’m not assuming Christianity false from the outset (which would beg the question). And so what if some have found it robust? Others have found it not so. Or found other contradicting religions to be robust.

But it also assumes that atheism is a kind of epistemic neutral ground …: if one is an atheist, he/she can examine all worldviews without bias. Again, the problem is that this is false.

Again, this is not where I’m going. It’s not that atheism is neutral ground or the atheist is somehow in sole possession of the clear-seeing glasses. Rather, it’s that atheism is the null hypothesis. It’s the starting point. The theist is making the bold claim and so has the burden of proof.
I anticipate the “Yes, but most people have been theists throughout history” argument again. To that, I simply repeat: you make the bold claim, so you have the burden of proof.

We can’t observe quarks or black holes,
but we should see their effects.
We do.
We can’t observe the Christian God,
but we should see his effects.
We don’t.
— Victor Stenger, “Faith in Anything is Unreasonable”

 
Image credit: Eirik Solheim, flickr, CC

The idea behind this argument is that humanity has invented thousands of gods throughout history. The atheist and the Christian both reject these many gods, so rejecting gods can’t be something the Christian would object to. Why then complain when the atheist rejects that one final god? (Part 1 here.)
Sharpen your wits by seeing how you’d respond to these Christian rebuttals.
9. Courtier’s Reply

Unless one has made a serious study of philosophical theology as it has been developed within the Neo-Platonic, Aristotelian, Thomistic and other Scholastic traditions, one’s understanding of traditional Christian, Jewish, and Islamic theology, not to mention philosophical theism, is simply infantile. …
The [objection represents] a failure to understand even the fundamentals of the position one is attacking. (Source)

In other words: Christianity has had 2000 years to develop sophisticated theology, and by bypassing that, your “I just believe in one less god than you do” argument is beneath contempt. And don’t get me started on the uselessness of parodies like the Flying Spaghetti Monster.
This is a nice example of the Courtier’s Reply, a logical fallacy in which an opponent is declared at the outset unqualified to even enter the field of combat, let alone make a thoughtful contribution to the debate. (It was developed by PZ Myers in 2006 as the imagined reply by one of the Emperor’s self-important courtiers to the charge that the Emperor wore no clothes.)

What matters in evaluating classical theism is not what your Grandpa or your Pastor Bob have to say about it, but rather what serious thinkers like Aristotle, Plotinus, Athanasius, Augustine, Anselm, Aquinas, Avicenna, Averroes, Maimonides, and countless others have to say.

Your mistake is imagining that “serious thinkers” speak with one voice. By starting with one Christian perspective and then selecting scholars, you can hide behind their writings. But other Christian scholars could be collected to argue a different story, and each camp finds heresy in the thinking of the others. Add in theologians from outside Christianity, and the chaos increases.
That’s the power of the “one less god” argument. It bypasses not just the “sophisticated theology” of Christianity but that of all the other religions and challenges the Christian to justify why, if dismissing religions by the hundreds is reasonable, one religion must be kept.
10. Car analogy. 
Here is a thought experiment. Let’s say that you and I both own cars, and we use our cars almost every day. They’re essential to our lives, but we each need just one. We don’t own hundreds of cars. And then one day, you lose your car—say you’re in an accident and you can’t afford a new one. You wouldn’t tell me, “It’s no big deal—I just have one less car than you do.”
My first response would be to ask why this is a good analogy. We understand cars and how useful they can be. Show me that your god exists and is useful in the same way. They seem very different to me.
We can map this onto a world with Christians and atheists by imagining that you (the guy who has no car) switches over to mass transit, taxis, Uber, rental cars, Zipcar, and so on as appropriate. You get where you want to go, though in a different way, just like the atheist has answers to the big questions of life, though different ones than the Christian has. So, no—dropping your supernatural “car” isn’t a big deal. Just ask an atheist.
11. Arithmetic analogy

There are a theoretically infinite number of possible answers to the equation “Two plus two,” but only one actually true answer. To say that “Two plus two equals four” is to automatically make me an unbeliever in all the other possible answers. It’s not rational, however, for the atheist to say, “Well I just go one step further and choose to disbelieve that four is the answer either. (Source)

This source has deliberately chosen an example where “one” is the correct answer to “How many answers are valid?” I wonder if it’s a coincidence that “one” is also the answer the Christian wants from “How many gods are there?” While arithmetic problems always have one correct answer, the correct answer to “How many gods are there?” could be one or twenty or zero.
Note also that arithmetic has proved itself, but religion has not. The many incompatible religions look like they’re all manmade. Comparing religion to arithmetic is to illegitimately appropriate arithmetic’s success. (More in this analysis of the map of world religions.)
12. Who needs evidence? Not the Christian.
This argument cautions us to not assume that the Christian reaches his worldview as an atheist might. Don’t assume that the Christian has sifted through the evidence for the hundreds of other gods, found none, and concluded that his original Christian belief is correct. Instead, some Christians say they have had a personal revelation. They also point to “self-verification of the Holy Spirit within.”
On this, William Lane Craig adds,

It is the self-authenticating witness of the Holy Spirit that gives us the fundamental knowledge of Christianity’s truth. Therefore, the only role left for argument and evidence to play is a subsidiary role. (Reasonable Faith, Third Edition, p. 47)

Self-authenticating? Evidence in a subsidiary role? I hope you’re consistent and allow the other guy to have the same careless attitude toward reality.
(Craig takes this thinking much further, and I analyze that here and here.)
Concluded in part 4.

“What would you replace Christianity with?”
“When a man has smallpox, you don’t replace it with anything. 
You cure him and send him on his way.”
— Cross Examined

Image credit: Tighten up!, flickr, CC

The idea behind this argument is that humanity has invented thousands of gods throughout history. The atheist and the Christian both reject these many gods, so rejecting gods can’t be something the Christian would object to. Why then complain when the atheist rejects that one final god? (Part 1 here.)
Let’s continue analyzing Christian rebuttals to this argument.
5. The other gods weren’t really gods.

The gods of these pantheons were/are not really gods in the proper sense. In order to call them such is a misunderstanding of what “god” means. In other words, they were functional deities who carried a role that was expedient to the life and happiness of the people. They were the gods of rain, sun, crops, war, fertility, and the like. They were the “go-to” immanent forces who had no transcendence or ultimate creative power. They were more like superheroes from the Justice League than gods. (Source)

Sounds like Yahweh. He was the war god in the early Justice League of Israel Hebrew pantheon.
6. The other religions were polytheistic, and that doesn’t count.

I understand perfectly why [atheists] reject all the other gods. It is because they reject polytheism. But I don’t understand how this parallels to the rejection of the Christian God. It is a slight of hand to make such a comparison (effective as it may be). People believe in these two completely different things for completely different reasons and, therefore, must reject the two differently. The same arguments used against these gods cannot be used effectively against the Christian God. Once polytheism as a worldview is rejected, all the millions of gods go with it. I don’t have to argue against each, one at a time. (Source)

In the first place, what’s wrong with polytheism? This author gives no justification for any prejudice against polytheism. I’ll grant that it’s a primitive and superstitious view of the world, but then so is monotheism. (Let’s avoid the temptation to detour into the Trinity to discuss whether Christianity actually is monotheistic.)
If one wants to claim that the invention of monotheism was a bold innovation, Amun-Ra in the Egyptian pantheon was worshipped as the sole god before Yahweh was.
In the second place, the Old Testament idea of Yahweh evolved. The theology of the people who would become the Jews began as a pantheon like those of the cultures around them, and only gradually did Yahweh become the sole god.
7. It doesn’t count if that god wasn’t a creator god.

This first cause is by definition God. (Source)

Look it up. You won’t find “god” defined as the first cause. We’re not talking about the upper-case version.

Simply put, whoever started it all (the time, space, matter creation) is the only true God. … God, while able to interact and love mankind, must transcend all that we see and know. He must be outside of our universe holding it all together, not simply the most powerful actor in our current play.

Other religions from that part of the world had gods who created our world from the carcass of the defeated chaos monster. For example, Marduk, the god of the Babylonians, formed the universe from the body of Tiamat. This story was common enough to be given a name: the Combat Myth. We see hints in the Bible that Yahweh defeated Rahab (another name for Leviathan) in a way that parallels this creation story, which makes Yahweh’s creation story just one of a series of similar stories.
It’s almost as if the Old Testament god didn’t actually exist but was inspired by stories from the surrounding cultures …
8. God must be infinite in greatness, and nothing less will do.

Alvin Plantinga representatively captured the concept of God as a being “having an unsurpassable degree of greatness—that is, having a degree of greatness such that it’s not possible that there exist a being having more.…
There is no, and cannot be, a possible world with two or more beings that possesses unsurpassable degree of greatness. (Source)

Define God (that is, Yahweh) anyway you want, but that’s not what we’re talking about. We’re talking about the thousands of gods—that is, deities—that people believe in.
This author is saying, “What thousands of other gods? There is only one God by definition!” But that’s changing the subject. He’s imagines Yahweh as a privileged example of the set of gods, and we must decide if that privilege is warranted.
Another author provides insight into Plantinga’s definition of God as the greatest possible thing. “Greatest” sounds impressive until we try to define it—perhaps it’s debatable.

The resurrected Osiris asked Horus a question, “What is the most glorious deed a man can perform?”
Horus answered, “To take revenge upon one who has injured his father or mother.”

Are we all on the same page with Horus? If not, then greatest, most glorious, or best may be in the eye of the beholder. (The difficulty of finding the best is made clearer in the Ontological Argument.)
Continue with part 3.

In politics and in life, ignorance is not a virtue.
It’s not cool to not know what you’re talking about.
That’s not keeping it real or telling it like it is.
It’s not challenging political correctness…
that’s just not knowing what you’re talking about.
— Barack Obama, commencement speech at Rutgers

Image credit: Takashi .M, flickr, CC

I like to study arguments on both sides of the God question. The idea behind “I just believe in one less god than you do” is that humanity has invented thousands of gods throughout history. The atheist and the Christian both reject these many gods, so rejecting gods by the truckload can’t be something the Christian would object to. Why then would the Christian criticize when the atheist rejects that one final god?
Some Christians aren’t impressed. To make sure atheists use only arguments that can be supported, let’s look at rebuttals from Christian authors.
1. You’re misusing the word “atheist.”
The first concern is that the word “atheist” is used too broadly. One form of the argument is, “We are all atheists with respect to other religions, and we atheists just take it one step further.”

The core problem is calling Christians (and others) atheists! …
It would be absurd for a Christian to see a Muslim and say “Oh, they’re an atheist!” For the Muslim is clearly not an atheist, rather, he/she is a theist! (Source)

Does this author propose to now turn on his heel and walk away, confident that he’s slapped down the challenge? Surely the point is obvious, and if it is stated incorrectly, he should correct it. To do otherwise is cowardly.
I’ve written about a similar problem, the (supposedly) self-defeating statement. I’m frustrated at apologists who point out a logical or technical error in a statement and then sit back, as if that were a proper rebuttal. Yes, sometimes it is, but it is at least uncharitable to not consider ways to correct it.
Let’s hope the arguments get stronger.
1b. There’s a big difference between an atheist and a theist
For a related angle, start with this well-known version of the argument from Stephen Roberts: “I contend that we are both atheists. I just believe in one fewer god than you do.” Greg Koukl twisted it by imagining saying to a married man: “I contend that we are both bachelors. I am just married to one less woman than you.”
Someone is a “married man” whether he’s married to one woman or dozens. One’s bachelor status requires him to be married to zero, just like one’s atheist status requires one to accept zero gods.
Like argument #1, this is just a semantic argument. I agree with Koukl that the focus here is on the one (or few) woman you are married to, not the billions you’re not married to. His parallel to our statement is clear: the theist’s focus is on the one or more gods you believe in, not the myriad you don’t.
This handwaving does nothing to respond to the point that the Christian, like the atheist, happily denies the existence of all gods … but the Christian makes one exception. Why is the exception justified rather than special pleading? Doesn’t the Christian have the burden of proof to justify their position?
2. But pretty much everyone throughout history has believed in God!
Yeah, and pretty much everyone throughout history has believed in superstition. They might’ve seen evil spirits behind a death or a famine; they might’ve seen good spirits behind a military victory or good harvest. The longevity of those superstitions doesn’t mean that they’re correct or that we should privilege them in any way. And when we evaluate them, they don’t hold up. Does Christianity hold up any better?

While they differ on the specifics of what God is like … if everyone has some idea of God, then our question should be what is God really like, not does God exist. (Source)

In the first place, belief in god(s) has been widespread, not belief in God (Yahweh). And in the second, different religions can’t even agree on how many god(s) there are, what their names are, or how to placate them. The amount of harmony here is meager. Sorry—if you can’t agree on the fundamentals, that looks like just another superstition.
3. God is like a person about whom we have differing input. That’s no reason to doubt her existence.
This argument compares the world’s many competing religious views to the many competing views of a particular woman. Person A is married to her. Person B is her subordinate at work. Person C is an ex-boyfriend. Person D is a pen pal and has never met her. These and many other people know her in different ways and would have differing and perhaps even contradicting stories to tell, but an outsider wouldn’t conclude that she doesn’t exist.
This is true, but the analogy is poor. This is all commonplace. We know about people and about the many ways we can incompletely understand them. A claim about the supernatural (which has yet to be demonstrated) is a very different thing. “Suppose that a woman exists; now suppose that a god exists” doesn’t work.
A story about a woman that claims to be factual? The default is that that refers to an actual person. When that assumption turns out to be wrong, we have a man-bites-dog story, like the mysterious girlfriend of Notre Dame football player Manti Te’o. That is a bit closer to the god claim, but even there we’ve got a long way to go.
4. Christianity is real while the other religions from history were mere social conventions.

People did not really believe in Shu, Nut, Hercules, Baal, Wearisomu, Enki, Utu, Diana, and the like in the same way that people believe in Yahweh. Their belief was more of a social convention which included all the pressures that such a system demanded. Their gods were more “faddish” than anything else. (Source)

Remember how the god of the Old Testament changes. God was just a dude who walked in the Garden in the cool of the evening to chat with Adam and Eve, but later he said, “No one may see me and live.” God had to send out spies to get intelligence about the status of Sodom and Gomorrah, but now he’s omniscient. Who’s faddish now?

Their existence was rather fluid, changing and even morphing into other gods.

That reminds me of how the omniscient and unchanging Yahweh demanded rituals for the Hebrews to follow. Later, he threw out that rulebook and said that belief in the saving sacrifice of Jesus was the way to please him. You’d think he’d get it right the first time.
Continue with part 2.

“I don’t understand how you don’t believe in God.”
Well, you know how you don’t believe in Zeus?
Like that.
— Ricky Gervais

Image credit: Chris, flickr, CC