If you ask someone to define "faith" you will likely hear something along the lines of "Believing something without evidence" or "Belief without proof." There are traditions in both religion and philosophy that accept such a view. However, there are other understandings of religious faith that take evidence to be important. That is, many hold that a credible faith is a rational faith. Faith and reason are not only consistent, on such a view, but in fact faith in some sense depends on reason.
First, there are those who think that faith and reason are unrelated. The name for this school of thought is fideism
. This is the view that faith neither depends on reason nor is it based on evidence. Instead, it is a non-rational belief, or perhaps even an irrational one. As Kelly James Clark puts it in his description of fideism, those who hold the view believe that if reason is opposed to faith, then "so much the worse for reason."*
In contrast to this, evidentialism is the view that faith must be rational. That is, a sound religious faith must be based on good evidence and arguments. For those who accept some form of evidentialism, the key issue is whether or not there is good evidence for the existence of God. A theist might offer some of the classical theistic arguments or more contemporary ones in support of this claim.**
Skeptics question the validity of such arguments, and raise arguments against the claim that God exists. They may argue that the existence or amount of evil and suffering in the world shows that God does not exist, or that naturalistic evolutionary theory is sufficient for explaining our origins. Theists reply with arguments that seek to show why God might allow the evil and suffering that exists in the world, or they may claim that the limits of evolutionary theory related to explaining certain aspects of reality (such as consciousness or moral values) reveals the need for God to explain reality as such.
At any rate, these are controversial and often difficult questions. The important point, though, is that it is a mistake to define faith as opposed to reason and then argue that all who possess some sort of religious faith are thereby irrational. Rather, we should examine the arguments offered for and against the existence of God on their own merits. If we simply appeal to a definition of faith as belief without evidence, we close ourselves off to exploring the possible evidence on each side of the God question.
Finally, at least in the Judeo-Christian tradition, there is another important element to faith. Faith is not mere belief in the claim that God exists. Just believing a statement has little to do with one's life, in many respects. The New Testament takes faith to include belief, but it goes beyond this as well. Faith also includes trust, in this case trust in God. So perhaps the best definition of faith is something like this: Faith is trusting in God, based on sound reasons.
*See Kelly James Clark, Readings in Philosophy of Religion (Broadview Press, 2008), p. 188.
** For example, see www.reasonablefaith.org