What is faith? Is it belief in accord with the evidence? Is it belief regardless of the evidence? Something else? Faith is defined in many ways. Let’s try to untangle the confusion (some of which I suspect is deliberate).
Editors’ Note: This article is part of the Patheos Public Square on Faith and Reason. Read other perspectives here.
Faith and Mother Teresa
Mother Teresa’s troubled relationship with faith is well known. She was celebrated by society but ignored by God. About her prayer life, she wrote of “silence and emptiness.” She described her own life as “darkness,” “loneliness,” and “torture” and compared it to hell. An editor at Jesuit magazine said, “I’ve never read a saint’s life where the saint has such an intense spiritual darkness. No one knew she was that tormented.”
And yet one biographer said about this dysfunctional life, “Her church regarded her perseverance in the absence of a sense of divine response as perhaps her most heroic act of faith.”
Heroic? When God doesn’t answer, is he inscrutable or just not there? Was Teresa displaying admirable perseverance or foolish futility? This persistence is laudable only in a world where religion celebrates faith over evidence.
For being so widely used, the definition of “faith” can be slippery. Let’s consider the two popular definitions, each staking out a different relationship with evidence.
Faith definition 1
Everyone wants good reasons supporting their beliefs—or at least to appear that way. Many Christians use the following definition for “faith.”
Faith definition 1: evidence-based belief; that is, belief that follows from the evidence. For example, you might have faith in your car’s reliability because it’s done a great job so far, but that faith will fade if it begins to act up. I would call this “trust,” and many Christians are fine with that—they just say that “faith” and “trust” are synonyms.
The Bible has plenty of examples where evidence backs up belief.

• Elijah challenged the 450 prophets of Baal to a bake-off where the first one to get his sacrifice lit by heavenly fire gets to execute the others (1 Kings 18).
• An angry crowd came to Gideon’s house after he destroyed an altar to Baal. Gideon’s father told them, “If Baal really is a god, he can defend himself when someone breaks down his altar” (Judges 6:31).
• The parable of the sheep and the goats (Matthew 25) makes clear that good works and not faith are the ticket to heaven.
• Jesus did his miracles in part to prove his divinity. “Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the works themselves” (John 14:11).

This conflating of faith with trust is popular among modern apologists as well.

• Mathematician and apologist John Lennox said, “Faith is not a leap in the dark; it’s the exact opposite. It’s a commitment based on evidence.”
• Christian podcaster Jim Wallace said that faith is “trusting the best inference from the evidence.”
• Presbyterian leader A. A. Hodge said, “Faith must have adequate evidence, else it is mere superstition.”

Drs. Norm Geisler and Frank Turek in I Don’t Have Enough Faith to Be an Atheist said that plenty of evidence backs up Christian claims:

[For many nonbelievers] it’s not that they don’t have evidence to believe, it’s that they don’t want to believe.
God has provided enough evidence in this life to convince anyone willing to believe, yet he has also left some ambiguity so as not to compel the unwilling.

(My post responding to this book is here.)
Faith definition 2
But if you have any familiarity with Christianity, you know that doesn’t cover the spectrum. Faith can also have a very different relationship with evidence.
Faith definition 2: belief held not primarily because of evidence and little shaken in the face of contrary evidence; that is, belief neither supported nor undercut by evidence. This would be a belief that can’t be shaken by a change in evidence (such as, “I won’t give up my faith in Jesus for any reason”). Evidence for one’s belief can be nonexistent or it can argue against one’s belief (blind faith), or evidence can simply be insufficient.
Again, let’s start with the Bible to find support for this evidence-less faith:

Now faith is being sure of what we hope for and certain of what we do not see. … And without faith it is impossible to please God, because anyone who comes to Him must believe that He exists and that He rewards those who earnestly seek Him (Hebrews 11:1–6).
Then Jesus told [Doubting Thomas], “You believe because you have seen me. Blessed are those who believe without seeing me.” (John 20:29)

The Hebrews passage has no need of evidence, and the statement of Jesus celebrates those who believe despite a lack of evidence.
Let’s check in with some early church fathers.

If you chance upon anything [in Scripture] that does not seem to be true, you must not conclude that the sacred writer made a mistake; rather your attitude should be: the manuscript is faulty, or the version is not accurate, or you yourself do not understand the matter. (Augustine)
[I don’t understand to believe but rather] I believe to understand. (Anselm of Canterbury)

Now consider some modern sources. Kurt Wise has a PhD in geology from Harvard, and yet he’s a young-earth Creationist. In high school he used scissors to cut from a Bible everything that science concluded couldn’t be interpreted literally. He said about the resulting corrected Bible, “I found it impossible to pick up the Bible without it being rent in two.”
But his definition of faith doesn’t follow the evidence:

If all the evidence in the universe turns against creationism, I would be the first to admit it, but I would still be a creationist because that is what the Word of God seems to indicate.

William Lane Craig’s gullible acceptance of magic rather than evidence as the ultimate authority is equally disturbing:

Should a conflict arise between the witness of the Holy Spirit to the fundamental truth of the Christian faith and beliefs based on argument and evidence, then it is the former which must take precedence over the latter, not vice versa. (Reasonable Faith [Crossway, 1994] p. 36)

We can see both definitions of “faith” in Geisler and Turek’s I Don’t Have Enough Faith to Be an Atheist. Above, we saw how they celebrate evidence when they think they have it. But the very title of their book denigrates “faith” as a leap unsupported by evidence. They say:

The less evidence you have for your position, the more faith you need to believe it (and vice versa). Faith covers a gap in knowledge. (p. 26)

Finally, consider a faith that has real-world consequences. Though religion wasn’t involved, it seems faith rather than physics guided a hot-coal-walking exercise put on by motivational speaker Tony Robbins in 2012. Twenty-one people were treated for burns.
Snake handlers believe that Jesus said about them, “In my name … they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all” despite the very clear evidence to the contrary.
Pastor Mark Wolford died from a snakebite in 2012, and he had watched his father die from the same thing. Pastor Jamie Coots refused medical treatment for a snakebite in 2014 and also died. If anyone knew that God doesn’t protect believers from snakebite it was him, since that was his ninth snakebite.
Christian commentary
Christian scholars grope around as they try to justify belief without evidence.
John Warwick Montgomery suggests crossing a busy street as a parallel. You never have absolute certainty of your safety when you cross a street. Instead, you wait until you have sufficient confidence, then you cross. And then, you don’t just take 99 percent of yourself across (to match your degree of confidence in the safety of the trip); you take all of yourself. Faith jumps the gap, both for busy streets and for Jesus.
Another example is marriage. You don’t have certainty that the Bible is true, but you don’t have certainty that you’ve picked the right marriage partner, either.
Nope. Neither example makes the Christian case. Crossing a street is always based on evidence. You look for good evidence that it’s safe, and you reconsider your conclusion if new evidence comes in. You also weigh evidence in the search for a compatible mate. In the same way, we follow the evidence for the reliability of the Bible as well—and find very little, not enough to support its enormous claims.
Alvin Plantinga has an interesting angle:

No one thinks there is good evidence for the proposition that there are an even number of stars; but also, no one thinks the right conclusion to draw is that there are an uneven number of stars. The right conclusion would instead be agnosticism.

Is there a reason to believe that there’s an even number of stars? No. An odd number? No. What about God—is there reason to think that he exists? No. That he doesn’t? Yes! You can throw up your hands in the case of the number of stars because it’s impossible to answer—agnosticism (or apathy) is an appropriate response. But the data is in for God, and that hypothesis fails for lack of evidence, just like the leprechaun and Zeus hypotheses.
Anselm said, “I believe to understand,” but that won’t work for me. If God exists, he gave me this big brain to use. It would be impolite to ignore its objections or be a Stepford wife. If God exists, he’d be happy to see me challenging empty Christian claims.
Pick a definition and stick with it
Lots of words have multiple definitions. The problem here is that “faith” is often used to mean belief based solidly on evidence (but only when outsiders are looking). For example, “I have faith that the sun will rise tomorrow.”
But within Christian circles, the heavy burden of evidence is shrugged off, and faith might mean “believing as your heart speaks to you.” Christians might then speak with unjustified confidence about what heaven is like and who’s going there, what signal God conveyed with a recent disaster, who’s on God’s naughty list, and so on.
Christians, to help you make your own arguments more clearly and honestly, let me suggest some word hygiene. Use trust to mean evidence-based belief, belief in accord with the evidence and which will change as the evidence changes. Use faith to mean belief not primarily supported by evidence and which is not shaken by contrary evidence.
Each word has its place. Be consistent. Sloppy usage only confuses your message and yourself.
Continue with “How Reliable Is a Bridge Built on Faith?” for a spirited critique of faith.

Faith is the excuse people give
when they don’t have a good reason.
— Matt Dillahunty, Atheist Experience