Let’s conduct an experiment to find the better of two alternate worldviews. First is Christianity/theism. Opposing that is naturalism, the belief that natural explanations are sufficient to explain the world we see.
Each worldview makes predictions about how our world should look. We’ll consider those predictions and compare them against the evidence from reality to see which worldview does the better job.
God’s hiddenness
Theism, by the definition that I’m using, proposes supernatural deit(ies) that engage with humanity. (Contrast that with deism, in which a deity could’ve started up the universe and then walked away.)
Our example of theism is Christianity, which tells us that God should be obvious: “God’s invisible qualities … have been clearly seen, being understood from what has been made, so that people are without excuse” (Romans 1:20). It makes the same claim when it says that God is anxious to have a relationship with us, since understanding God’s plan and putting our faith in Jesus is mandatory for us to escape hell.
God should be obvious, and his message to us should be unambiguous. But when we look around, there is no good evidence for God, just as naturalism predicts. (I discuss this Problem of Divine Hiddenness in detail here.)
Fragmentation into sects
Even if humans invent religions, theism predicts that clear evidence for the one correct religion would outshine all the rest. Invented religions might always be background noise in the religious environment, but the correct religion—the only one actually supported by evidence of real god(s)—would quickly spread from its introduction thousands of years ago to become and remain the biggest religion by far. There would be no contest to any unbiased observer which religion was correct (more here).
That’s not what the evidence shows. For all of human existence, the majority of people at any moment have always had the bad fortune to not believe in the one correct religion (assuming there is one). Religion is a cultural phenomenon. Christianity is the largest religion at the moment, though it might not be in fifty years when Islam is expected to become number one. Christianity is not the oldest religion, it didn’t become largest until centuries after its founding, and it’s never held the majority of the world’s people.
A single, unambiguous message doesn’t exist even within Christianity. There are 45,000 Christian denominations, a number that is growing rapidly.
The Bible itself documents how God’s fundamental properties have evolved.

• God was initially just a guy who walked in the Garden to chat with Adam and Eve, but later he said, “No one may see my face and live” (Exodus 33:20).
• God had to send out agents to get intelligence about Sodom and Gomorrah, but ask Christians now, and they’ll say that he’s omniscient (“New and improved God 2.0—now with 10²⁰ times more omniscience!”).
• God was initially part of a pantheon, and only later do we get a clear statement of monotheism (Isaiah 43:10, for example).
• God was initially merely powerful, and he had limitations. Now we hear he’s invulnerable.

The map of world religions shows that religious belief doesn’t change with evidence like science does. Instead, it’s part of culture.
Relationship to science
Theism predicts that sacred texts would be useful in the real world. They wouldn’t be full of just-trust-me-on-this demands. Instead, they would be grounded in the real world so that we could see that their claims are both surprising (far beyond what was known in that society at the time) and reliable. We wouldn’t need faith to accept the supernatural; it would be obvious that this wisdom didn’t come from any human society.
Christianity is again a counterexample. Any scientific statement within the Bible that’s true was known by the culture that produced that part of the Bible, and all other scientific claims within the Bible are false. Mining the Bible to find verses that vaguely anticipate modern scientific discoveries is a popular hobby, but science has learned nothing about reality from the Bible.
You’d think that the Bible would at least have room for simple science that would greatly benefit people. For example, how about a recipe for soap plus basic hygiene rules? It would only take a paragraph, but we find nothing. Even Jesus’s healing miracles are just superstitions of the time.
In the competition between science and religion, “God did it” was the answer for famine, plague, drought, disease, and even war in centuries past. Dogma rather than evidence pointed to God, and science has steadily produced reliable answers to replace God for countless scientific puzzles. The reverse has never happened. Shoehorning God into the remaining puzzles makes him a “god of the gaps,” a pitiful rearguard action that makes a joke of the all-powerful Creator of the Universe.
At best, apologists can say, “Well, science hasn’t answered this question,” unconcerned that Christianity hasn’t answered any question. Yes, science does have unanswered questions on its to-do list—that’s how science works. These aren’t questions that theologians have pointed out but are mostly obtuse questions that only science could raise.
Life
Theism predicts that life is designed and that life is the purpose of the universe.
Neither is true. A Rube Goldberg machine and a Swiss watch are both complicated but in different ways. Cells are complicated, but they’re more like the redundant and inefficient Rube Goldberg machine than the elegant watch. Designer-less evolution is sufficient to explain why life looks the way it does.
DNA is often cited as being so software-like that it must have come from a mind, but I argue that the sloppiness in DNA alone is enough to defeat the Design Hypothesis (the argument that life must’ve been designed).
The theist will say that software invariably comes from minds, but they forget that minds invariably come from brains. So where is the brain that houses God’s mind? And, no, software doesn’t invariably come from minds. Software can be evolved in a computer where it is randomly changed and tested for fitness, analogous to what happens in the real world to DNA.
The theist must look at the hundred billion galaxies in the universe, each with a hundred billion stars, and say that all of that is there because of humans on one planet in an insignificant backwater of one galaxy. Naturalism gets it right when it predicts no design and sees life as just something that happens now and then.
We’ll conclude this worldview comparison in part 2.

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: Image Catalog, flickr, CC