When accepting a statement as true, there are two basic methods. The first is reason. It is when the known evidence points to the statement being true, and when the truth of the statement doesn't contradict other knowledge. The second is faith. It is when one accepts a statement as true without evidence for it, or in the face of evidence against it.
There's a lot of confusion about what exactly faith is. Many people confuse belief with faith. It's said that if you believe something, you must be taking it on faith. This is a denial of the fundamental distinction between reason and faith. It pretends that evidence for or against an idea is irrelevant.
The result of using faith consistently is the complete inability to think. Without any criteria for accepting a statement as true, every random idea, whether true or false, would be just as likely to be accepted. Contradictions would exist. No higher level abstractions could be made. Faith nullifies the mind. To the degree ideas are taken on faith, the process of thinking is subverted.
Are there any ideas we take on faith? As a friend once asked, if we've never been to Afghanistan, how do we know it actually exists? Even if we were to meet people from Afghanistan, they could always be lying. This is taken to be an act of faith, since we have no direct evidence for the existence of Afghanistan.
This is mistaken, though. The evidence we have for accepting the existence of Afghanistan does exists. The evidence is based on the knowledge that other people have shared. First, there is universal acceptance of the fact that it exists. It is possible that everyone on the planet is lying, but there is no evidence for that claim. Also, there is reason to believe that if Afghanistan didn't exist, people from the bordering countries would say so. And since satellite imagery shows that there is land there, and the area around it is occupied, it is reasonable to assume that land is occupied as well. Furthermore, there is absolutely no known evidence that it doesn't exist. There is no known motive for the entire world to try to trick us. So in fact, the evidence we have suggest it does exist. Acceptance of it is an act of reason.
There's an important distinction here, though. When we accept the evidence from others, we must have reason to believe that they know the truth. In the case of Afghanistan, I mentioned bordering countries. But there are people who claim to have been there, or that lived there.
Other cases are fundamentally different. When someone claims to have supernatural knowledge, or the ability to gain knowledge in a way that you are unable to, their claims cannot be considered valid. If someone claims to be able to speak to their god, and tells you what god demands, you have no reason to accept it as true. In fact, it should be rejected. If he claims to have knowledge which you are incapable of achieving, his beliefs must be rejected. If one has to accept the knowledge of others, he must use reason in order to decide which others to listen to. Again, if there is no evidence or contrary evidence for accepting a person's beliefs, it is not an act of reason. It is an act of faith.
Faith is an act of mental destruction. If there is no evidence for a claim, then accepting it is irrational. It is more likely to be false then true (since there are more false ideas then true ones, being that their is only one reality). Building a structure of knowledge on such a flimsy foundation will leave it shaky and unstable. Eventually, even if confronted with evidence against it, one's mind will be so dependent on the belief that fear of one's world view collapsing will encourage one to reject the evidence. When this happens, one acts against reality. This is an act of destruction.