Ask an agnostic or atheist what they think faith is. Faith is believing without evidence, they'll say.
It should be clear from this definition (I think so anyway) that faith is not at all a virtue, and may in fact be a vice. If our beliefs are not constrained by evidence, then we could, in principle, believe anything, anything at all. Only a few of those possible beliefs would be correct, only a few would be good to believe in. And since there is no evidence, we don't know which ones are the correct beliefs. So why are we playing this guessing game? And why should it be a virtue to play the guessing game?
In practice, though, this is a rather ineffective argument against Christianity. A lot of people will simply say, "You just don't get it, do you? That's not what faith is at all." Which is potentially a good point. I wouldn't want to be constructing some sort of straw man of faith. So let's talk about faith as Christians use and define it, not how atheists and agnostics define it.
The thing is, Christians have a rather ... confused (for lack of a better word) understanding of what faith is. I suppose a more positive way of putting it is "complex" or "diverse". I think it's a fairly common sermon topic to explain yet another point of view on what faith really means. The number of ways to understand faith is perhaps as large as the number of Christians, possibly larger.
Allow me to explain what faith meant to me a long time ago, back when I was still Christian. When I was Christian, I was taught that Christianity does not require a leap of faith. There are, after all, many arguments we could use in favor of Christianity. None of these arguments are particularly effective by themselves, and absolute proof can never be achieved. They only allow us to get closer and closer to God by degree, never fully reaching him. Therefore, if we ever want to reach God, we have to cross this little gap. Faith is that which gets us across that gap.
A lot of you probably think this concept of faith is really unusual or bizarre. Which is the point. Everyone seems to have a different idea of what faith is.* Faith is our gift to God. Faith is a gift from God. Faith is a relationship. Faith comes from a feeling deep within. Faith is supernatural assent. Faith is an experience. Faith is trust. Faith is confidence. Faith is an attitude. Faith is the will to believe. Faith is believing in things not seen. Faith is what gives us certainty. And yet, faith can coexist with doubt. Faith can coexist with evidence.**
*It would be hilarious if another "agnostic free mason" told me off because I can't appreciate the monolithic nature of true Christianity.
**Most of these ideas come from a list of definitions collected by Greta Christina. Greta took the definitions from religious sources.
Well, how could I possibly attack this huge amorphous blob of "faith"? By chopping into smaller pieces, of course. Allow me to define two kinds of "faith".
Faith-1 is any kind of faith which involves arriving at beliefs by circumventing the proper routes to knowledge. For example, believing without evidence is faith-1. Jumping the gap from uncertainty to certainty is also faith-1. Subtly "pushing" oneself in one direction towards belief is faith-1.
Faith-2 is everything else. For example, trusting someone you know (assuming it really is someone you know) is faith-2. Having a positive or optimistic outlook on life is faith-2. Believing in something which you have evidence for is faith-2.
This is my position: Faith-2 is perhaps justifiable, but faith-1 is not justifiable at all. Most religious believers have both faith-1 and faith-2, and may not necessarily see them as distinct. In fact, that's a major problem, that they do not usually distinguish between the two. They can argue for faith-2, which sounds very reasonable, and then later switch it around for faith-1. It would be very difficult to defend faith-1 directly, so people often cheat.
I myself could say that I have faith-2, but I would strongly prefer not to call it faith at all. I do not want to be complicit in this confusion of faith, this common equivocation. I have trust in my friends (the ones who are reliable anyways), not faith. I believe, or accept most established scientific theories because the evidence is inconsistent with the alternatives.