It's far too common to see religious theists trying to defend their beliefs by relying on faith, claiming both that faith justifies their position and that their beliefs are based on faith. Skeptics and freethinkers are justified in regarding this as little more than a cop-out because faith isn't really any kind of standard that can be tested for reliability. Even if religious theists don't intend it in this manner, it seems that in practice "faith" is simply pulled out whenever attempted arguments based on reason and evidence fail.
There are numerous problems with trying to justify any belief, philosophy, or religion on faith. The most significant may be the fact that there is no good reason for only allowing a single religious group to use it. If one person can offer it as a defense of a religious tradition, why can't a second person use it to defend an entirely different and incompatible religious tradition? Why can't a third person use it to defend an incompatible, secular philosophy?
So now we have three people, each defending completely different and completely incompatible beliefs systems by claiming that they are justified by faith. They can't all be right, so at best only one is right while the other two are wrong (and it may be that all three are wrong). How do we determine which, if any, is correct? Can we construct some sort of Faith-o-Meter to measure which one has the True Faith? Of course not.
Do we decide based on whose faith is the strongest, assuming we can measure that? No, the strength of a belief is irrelevant to its truth or falsehood. Do we decide based on whose faith has changed their lives the most? No, that's no indication of something being true. Do we decide based on how popular their belief is? No, the popularity of a belief has no bearing on whether it's true or not.
We seem to be stuck. If three different people each make the same "faith" argument on behalf of their beliefs, we have no way to evaluate their claims to determine which is more likely correct than the others. This problem becomes more acute, at least for religious believers themselves, if we imagine that one of them is using faith to defend an especially heinous belief system — like, for example, one that teaches racism and anti-Semitism.
Claims about faith can be used to justify and defend absolutely anything on an equal — and equally unreasonable — basis. This means that faith ultimately justifies and defends absolutely nothing because after we're done with all the faith claims, we're left precisely where we were when we started: faced with a set of religions that all appear to be about equally plausible or implausible. Since our position has not changed, faith obviously added nothing to our deliberations. If faith added nothing, then it has no value when it comes to evaluating whether a religion is likely true or not.
What this means is that we need some standard independent of these religions themselves. If we're going to evaluate a group of religions, we can't rely on something internal to just one of them; instead, we must use something independent of them all: something like the standards of reason, logic, and evidence. These standards have been amazingly successful in the realm of science for separating the theories which are likely true from those which turn out to be useless. If religions have any connection to reality, then we should be able to compare and weigh them against each other in at least a similar manner.
None of this means, of course, that no gods can or do exist or even that no religions can be or are true. The existence of gods and the truth of some religion are compatible with the truth of everything written above. What it does mean is that claims about the truth of religion or the existence of some god cannot be defended to a skeptical nonbeliever or freethinker on the basis of faith. It means that faith is not an adequate or reasonable defense of any belief or belief system which purports to have any empirical connection to the reality which we all share. Faith is also an unreliable and irrational basis for singling out one religion and claiming that it is true while all other religions, as well as any competing secular philosophies, are false.