There is a great deal of misunderstanding among atheists and agnostics about the nature of religious faith. A common view is that faith is just another name for credulity, a willingness to believe in what are effectively fairy tales. This is not the case, as I hope to show.
In fact religious people mostly come by their beliefs in the same way as scientists come by theirs: they evaluate the evidence and, where conflicts occur, decide which bits of that evidence to prioritise. That's hardly surprising, since in practice religious people and scientists are often the same people.
It is helpful to think of belief as something that exists between two extremes of certainty. On the one hand, there are things we know to be true: no one says he believes that the earth is round. On the other hand, there are things that we know are not true: no adult believes in Santa Claus.
But the really interesting questions, including those on which juries have to decide, invariably involve conflicting evidence. In such cases, we must weigh the evidence, and some of it will have to be rejected in order to resolve the conflicts. What we reject is going to depend on our existing beliefs, our values, and our ideas of what is likely to be true and who is likely to be trustworthy. Thus two equally intelligent and honest people may well come to opposite conclusions on the matter.
When someone says "I believe", it is really short for "I believe this to be the case but I recognise that other people may come to a different conclusion based on the same evidence". That is certainly not an irrational attitude, whatever Professor Dawkins may think. What does seem to me to be irrational is to say that you know there is no God, when no fallible human being could possibly know such a thing for certain.
The difference between religious and scientific beliefs is not in the way they are acquired, but in what happens next. If I come to believe that birds evolved from dinosaurs, that is certainly very interesting but it is hardly life-changing. If I were later to discover that birds actually had a completely different ancestry, it would not matter all that much. My beliefs about such things can therefore be allowed to fluctuate with the ebb and flow of the evidence.
If, however, I come to believe that a wise, holy and loving God created me and wants to develop a personal relationship with me, that certainly is life-changing! It immediately demands of me an attitude of personal trust, just as a new friendship or a marriage would do. And if I respond with such trust, that will inevitably affect how I am going to evaluate evidence on this subject in the future.
When Christians say that a relationship with God depends on faith, they are not saying that religion works by different rules, so as to justify a lower standard of proof. On the contrary, they are saying that relationships with God work by exactly the same rules as any other kind of relationship. All relationships are based on trust, and trust cannot be at the mercy of every random change in circumstances. If you trust a friend only when the momentary balance of the evidence seems to be in his favour, then you don't really trust him at all and he probably won't be your friend for long!
Unfortunately trust is a two-edged thing. The person who demands it of you may indeed be a potentially lifelong friend, but he could also be a confidence trickster who wants to part you from your money. The only reliable way to avoid being conned is to trust no one. But if you take that road, you will have no friends either.
With God, there is a further wrinkle, which explains why we need a completely new word to describe what is going on here. With a human friend or a spouse, there is at least no doubt of their existence, only of their benevolence. But when we have decided to believe in the existence of God (by considering and rejecting the evidence to the contrary as less reliable than the evidence in favour), the evidence we have rejected does not go away; it simply changes its coat.
What previously looked like evidence against God's existence ("If there were a God, He wouldn't have let the Boxing Day tsunami happen") now reshapes itself seamlessly into evidence against His goodness ("If God was really good and loving, He wouldn't have let the Boxing Day tsunami happen"). The mind's belief in God's existence and the heart's trust in His goodness turn out to involve rejecting precisely the same contrary evidence. When we are dealing with God, intellectual belief and personal trust merge together and become psychologically inseparable. We call this unique fusion "faith".
One of the best illustrations of the meaning of faith that I know is Shakespeare's play Othello (though I must admit that I know it mainly in Verdi's magnificent operatic version). Religious faith is not exactly the same thing as personal trust, but here the latter acts as a kind of metaphor for the former. Othello trusts in the virtue and chastity of his wife, Desdemona, and as long as he does so, all her actions seem to prove that she is indeed a good woman who loves him. But when he stops trusting her, everything she says and does seems to prove that she is a strumpet who is having sex with another man. The point is that Desdemona's behaviour has not actually changed; what has changed is the way Othello interprets it. He puts another frame around the picture and it becomes a different picture altogether.
Furthermore this loss of faith is presented not as an enlightenment but as a tragedy. When Othello sees his wife through the eyes of faith, he sees the real Desdemona. When he loses his faith in her, he sees a completely fictitious woman. And yet, for the sake of this fiction, he destroys the real Desdemona and himself too. This is precisely what unbelief looks like when viewed through Christian eyes.
Admittedly, there are some Christians who seem to have a quarrel with facts discovered by scientists. I suspect that in many cases they are able to maintain this quarrel only because they don't know very much about science. For example, to be a consistent young earth creationist, you need to reject not just evolution but all forms of radioactive dating as well, and that ultimately means rejecting the whole of modern nuclear physics. But most Christians do see precisely the same facts as the atheists do. We just don't see the same picture.
For example, where an atheist sees in evolution only a meaningless interplay of chance and necessity (Jacques Monod's bitter claim), a Christian sees a great work of art created with the aid of a genetic algorithm. And just as an art expert can recognise the work of a particular artist by his brush strokes, so we can see familiar quirks that we have learned to associate with the Divine Artist, such as his passion for biological puns. Think of the dolphin, the fish and the icthyosaur, the extinct marsupial wolf and the placental wolf, triceratops and the modern rhinoceros. Some of these "puns" can be explained away as examples of convergent evolution but not all of them. Take, for example, the stinkhorn fungus (Phallus impudicus) which looks uncannily like an erect human penis but smells of sh*t!
The hippopotamus has been quoted as proof that God sometimes makes jokes; the stinkhorn proves that He occasionally enjoys making dirty ones!