Theists, and others, use the word "faith" to mean many different things. Faith can mean trust in someone for example. But when it comes to religious belief, faith means belief in something despite a lack of evidence (or, in many cases, belief in spite of overwhelming evidence to the contrary). Is this kind of faith rational?
"I don't believe in God because I don't believe in Mother Goose." - Clarence Darrow
It makes just as much sense to have faith in Mother Goose as it does to have faith in a god. Many theists think that faith in Mother Goose would be ridiculous while faith in God is not. This is because they have been conditioned, often from birth, to have faith in their god and not to have faith in the many other equally plausible magical creatures. When someone says that faith is the reason that they believe in something, what they are really saying is that they have no valid reason whatsoever for holding their belief, but they believe anyway because they feel like it. Faith is nothing more than wishful thinking.
If one person can use faith to justify a belief, another person can use faith to justify a diametrically opposed belief. For example, faith provides no way of discerning who is correct when someone says that they have faith that God created the universe and someone else says that they have faith that the Invisible Pink Unicorn created the universe. Since it is clear that faith cannot be used to reliably justify any belief, we have to conclude that faith is irrational. In fact, in the entire history of human civilization, the only methods that we have been able to come up with for determining whether or not some claim is true are reason and objective experimentation that is repeatable, i.e., the scientific method.
Some theists claim that it takes as much faith to be an atheist as it does to be a theist. Nothing could be further from the truth. An atheist's lack of belief in gods is not a result of having faith that gods do not exist, but a result of lacking faith that gods do exist. Some atheists don't just lack belief in gods, but actively believe that gods don't exist. This belief does not come from faith, however, but from reason and evidence. Rational freethinkers, whether they be atheists, agnostics or other, invariably reject faith as epistemological nonsense.
"Faith is the great cop-out, the great excuse to evade the need to think and evaluate evidence. Faith is belief in spite of, even perhaps because of, the lack of evidence." - Richard Dawkins
"It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence." - William Kingdon Clifford