Lewis does two chapters on faith so we're just going to follow his example.
So why would faith be considered a virtue?
Lewis goes on to ask the question in this way: "what is there moral or immoral about believing or not believing a set of statements? Obviously, I used to say, a sane man accepts or rejects any statement, not because he wants to or does not want to, but because the evidence seems to him good or bad. If he were mistaken about the goodness or badness of the evidence that would not mean he was a bad man, but only that he was not very clever. And if he thought the evidence bad but tried to force himself to believe in spite of it, that would be merely stupid."
I disagree with Lewis' word choice here, inability to believe something does not make one 'not very clever', especially if the evidence is not at all as clear cut as some would like us to believe. But he does hit on why you can consider faith a virtue in my opinion.
Here's the bottom line, as far as I am concerned with regard to the 'evidence' of any religious group: you choose to believe it or not. There are *always* arguments against and different explanations for *anything* that has been recorded as a religious event or miracle. We may not fully understand these reasons yet but they are always there. So an inability to believe that an account written down 6,000, 4,000 or 2,000 years ago is utterly factually correct given the authors limited knowledge does not make a person dull or stupid. I take 'scientific' proof of any religion to be ridiculous. You will never prove anything that way. Not ever.
People of faith *choose* to have faith. Yes, there's something about the human mind that wants to believe in things. I think it's tied into our imagination and sense of curiosity and wonder. We need there to be meaning to our existence. We need there to be something after death which is a place that we cannot explore or understand with just our minds as we are now.
So you choose to have faith and then you choose what to have faith in. And then you choose to continue to have faith. Faith is a stance that you take. And this is true whether we're talking about having faith in a god or faith in the laws of nature.
You can *know* that something is true, but believing in it takes a little more effort. For example, floating on water. As a child, you know that you can float on water if you lay out on your back because you have been told this and you have seen other people doing it. But there is always that point when it's you being leaned back (learning to swim) where panic wants to set in because part of you is not entirely convinced that the water is going to hold you up.
That's where you make the choice. You can either believe that the water will hold you up if you are still, or you can not believe it and panic. Your panic makes the water fail to hold you up, reinforcing your reason for panicking. :) Conversely, if you choose to believe and be calm, the water will hold you up and reinforce your choice to believe.
That's how faith works and that's why it can be counted as a virtue. Faith is not simply something that one has any more than patience or charity. You train yourself and are trained by the people around you to have those things. But in all of that you are making the choice. Virtues are things that we all think that we should have because they make us better people and make society work better. The downside of that is that we don't have them naturally, though they do tend to make society work more smoothly. We have to work at them. We have to train ourselves to them.
I have an issue with conflating faith with a 'moral good', of course. There's the implication in that way of thinking that people who lack faith are somehow morally corrupt or more likely to commit crimes. Which is not true, of course. But this helps me to see where the idea came from when atheism was emerging from the shadows that atheists were godless people who would rape and steal and murder without a seconds thought because people who had been raised in this mindset had a hard time understanding that people could be good without this particular virtue.