Aquinas tells us that human sociality is partly, maybe even largely, exhibited in our common epistemic project. Now, I think trust is absolutely central to this project. Trust is the glue that binds the individual epistemic projects into a joint project, and more generally that binds us in non-epistemic contexts. In particular, it is natural to trust others, and we always have a pro tanto moral reason to trust another. A failure to take another person's assurance of something as a reason to trust in the assured thing (a claim, a commitment, etc.) is a failure to treat the other person as a co-participant in the joint project, and is a partial denial of our common sociality.
So we always have pro tanto moral reasons to trust others. There may, of course, be overriders or defeaters for these moral reasons. Still, a habit of appropriately taking into account the moral reason to trust others because of our common sociality is a virtue. This virtue is balanced between mistrust and credulity, and we can call it "proper trust."
This, I think, makes it intelligible how faith can be a virtue. Faith is a species of deep proper trust in God—a proper trust so deep that it cannot be a work of nature. Still, like other theological virtues it builds on a natural virtue, in this case proper trust.
Interestingly, though, the theological virtue, unlike the natural, may well cease to be a mean. For there is no such thing as trusting God too much, as he is perfectly trustworthy. This is a feature faith shares with charity, which is a supernatural love for God. For while one can idolatrously overestimate the object of love for a creature, one cannot overestimate the object of love for God. So there is a sense in which charity also is not a mean (not an original view). I do not, at this point, know exactly what I should say about hope.
