I have often heard Christians argue that one must 'just have faith' and believe that God exists, that Jesus was his son, sent to redeem us for our sins. Indeed, even the famous Danish existentialist philosopher, Soren Kierkegaard, argued that one must 'take a leap of faith' - and not be concerned with the fact that religious belief is indeed irrational.
But let's consider a case. Suppose that you are accused of a murder. Suppose that you go to court, and the prosecution argues that you are guilty, and that no evidence is needed. Imagine that the judge brings down his gavel and says "Guilty as charged". When you appeal the judgment, and plead your innocence on the basis that there's no incriminating evidence whatsoever, further imagine that the judge shrugs and responds "I don't need evidence, I have complete faith that you are guilty".
Now let's turn to the case of religion. If the above scenario is unacceptable to you, why, then, would you consider it acceptable to say "I just have faith" in the case of God?
The problem with 'just believing' is that if you are ignoring evidence, you are basing your belief purely on other peoples' opinions. Take the example of what happens when you meet someone from another religious background. Let's say, Judaism. Suppose you're a Christian. Now, both you and your Jewish friend believe in a divine power; you call him 'God', your Jewish friend calls him 'HaShem'. You both agree that your beliefs are taken on faith. But now suppose you are conversing one day about the details of your beliefs, and you encounter the issue of Jesus. Your Jewish friend says that Jesus may have existed, but not been God's son. You disagree, and say that he is a blasphemer. How is this debate to be decided? Since your views are only based on faith, rather than evidence, neither of you can prove his case. The best you can do is repeatedly appeal to the authority of your respective religious texts. But that won't work, because that authority, in turn, depends on how loudly you shout at each other.
Personally, I find it impossible to believe most things without evidence. Of course, I take certain things on authority. So, for example, if a professor of geophysics writes a book that says that millions of years ago, the continents were joined as one, I feel inclined to accept it, since he is a professor, and as such, has done a lot of research into the matter. This is an example of the argument from appeal to authority. Officially, philosophers consider the argument of appeal to authority to be somewhat fallacious, but in fact, when researchers 'cite' each others' work in academic journal articles, they're really appealing to the authority of the person they cite. (And demonstrating respect for the originator of the cited idea). So it's not an appeal to authority that's the problem, per se. The problem comes in when the cited authority is dubious. So, for example, if I wrote a journal article about religion, and in it, I cited the opinion of my toddler, the reviewers would reject my article, since my toddler is not a credible or respected authority on matters religious.
This brings us to the question of why people 'just have faith'. Like a scientific researcher who believes what the professor of geophysics says about the truth about continental drift, a believer will accept the authority of his priest or Bible. Believers, then, are not so much taking their belief 'on blind faith' (which is redundant), but instead, they are taking their belief on authority. I said above that taking one's belief on faith is redundant. This is because to 'have faith' means 'to trust' or 'to believe'. So having belief based on faith, is really to have a belief based on a belief. Which says nothing about truth. It's all castles in the air.
So how does truth get established, then, if not on the authority of texts? What makes an authoritative source, authoritative? How do we know which books or persons' statements are legitimate, or true, or at the very least, well-attested-to? The answer is evidence. The more evidence a belief has to support it, the more probably true it is. There is a formula that philosophers and statisticians use to calculate this. It's known as [Bayes' Theorem](http://en.wikipedia.org/wiki/Bayes'_theorem). It goes something like this:
P(H|E) = P(E|H&k) x P(H|k) / P(E|k)
This reads: The probability of the hypothesis, given the evidence, is equal to the probability of the evidence, given the hypothesis, and some background knowledge, multiplied by the probability of the hypothesis alone, given some background knowledge, divided by the probability of the evidence, given some background knowledge.
In this case, the evidence is say, fossils, plate tectonics, DNA mutations, speciation, and other scientific facts. The background knowledge would include things like the laws of science, causation, and so on. The hypotheses will be things like Christianity or Judaism or Atheism. So, in this formula, we have to ask: what is the probability that Christianity is true, given the evidence, say, of plate tectonics and DNA mutation? The answer must be near-zero, since Christianity and the Bible mention none of these things. The same applies to any other religion. Thus, on this formula, Christianity and Judaism would both come out as equiprobable, since the physical scientific evidence fails to support either of them in any major way. (Whether the evidence supports atheism is a side-issue for now). But the key point is this: you cannot decide between two religions on the basis of evidence, unless those religions adduce some evidence. So, where is the evidence? An old book does not count. There are many old books. I have an old Superman comic. That doesn't prove that Superman exists. We need proper evidence: scientific evidence. So, for example, if every molecule, on close inspection, bore the Hebrew words 'made by HaShem', we'd have strong evidence for Judaism.
In [another post on Viewshound](http://www.viewshound.com/religion/2011/6/religion-cant-have-its-cake-and-eat-it-too-part-3-), I argue that believers sometimes try to appeal to scientific evidence, such as the remains of Noah's Ark. There are, as I argued there, two problems with doing so. Firstly, evidence of one part of a story only raises the Bayesian probability of that story being true; it does not prove it. Secondly, if believers feel entitled to help themselves to scientific evidence (anything measurable or visible is scientific evidence) - then they must also accept the scientific evidence against them. If this is right, then science is the arbiter of truth, not religion. Religion can only escape from this by appealing to blind faith. But blind faith is not really blind. It is based on the Bible, or other religious texts, which in themselves are not based on anything at all. You can't argue that the Bible's authority is based on God's, since faith in God is based on the Bible. Hence, to say that the Bible is supported by God, and that your faith is supported by the Bible, is circular. Since the Bible is putatively the proof of God's existence, God cannot be supported by it. No theory is self-supporting; that's circular.
But is religion even a scientific theory? I argue that it is. Religion makes predictions, it makes claims about how the world is. Moreover, if God made the universe, he must be able to have effects, and initiate causes, in our own segment of space-time. That means that God's effects and actions should be measurable, and detectable by science. If God exists, then evidence for him exists, and it can be weighed scientifically against the contradictory evidence. Saying that 'you just need faith' is pointless; because it's saying that your only piece of evidence is a self-supporting book. That's just not good enough. If I wrote in my diary that you murdered a chap called Jones, and Jones was indeed dead, my diary would not count as sufficient evidence in court that you were guilty. Any judge who took my repeated insistence as to your guilt as evidence of your guilt, would be quite incompetent. We should take the same stance on religion. Mere repetitions of slogans is not proof of anything.
The famous ['big lie'](http://en.wikipedia.org/wiki/Big_Lie) argument applies here. Just because someone repeats a slogan to you often, doesn't mean it's true. You should verify the facts with your own eyes. Go and see for yourself. Hold a fossil and think of Bayes' equation: is this fossil more likely to exist on the hypothesis of God, or is it more likely to exist on the hypothesis of evolution? Keep asking yourself that. And don't be tempted to add ad-hoc explanations, such as 'God made fossils to test our faith'. Ad-hoc explanations are explanations that don't follow logically from your premises ('God exists'). There's nothing, in 'God exists' that entails that God would plant fossils to test your faith. There is, however, plenty in the theory of evolution that would entail that fossils exist. That's how you test if a theory is true. And religion is just one of many theories.