In this part, I'm dealing with faith in terms of epistemology. "Epistemology" is one of those ten-dollar words that philosophers have coined by stealing from the Greeks. "Episteme" means either "knowledge" or "understanding", depending on how you want to read the relevant chunks of ancient Greek. So, "epistemology" is the study of knowledge. This means that the sense of faith I'm interested in is the one which treats faith as a way of knowing something about god or the divine or the supernatural. (Last part's arguments about the coherence of the supernatural as a concept notwithstanding.)
Faith can be used differently. It can be used to express an attitude of trust or confidence sufficient to justify action. That is, one can have faith in god by trusting that god will make sure things work out for the best, and act on that basis. This connection to action, though, makes this sort of faith really a matter of ethics, and so I'll deal with it when I get to religion and morality.
Faith can also be used to express a sort of feeling, such as a sense of confidence that there is a god or hope that god exists. This also clearly has nothing to do with knowledge. It's also worth noting that it makes faith a pretty random thing. As with most feelings, faith will come in and out of existence, and will vary widely between people. If that's what religious faith really is, then I don't see why it's worth worrying about at all.
The best version of faith I know comes from Immanuel Kant, and it goes something like this. On the one hand, there's what can be known -- the knowable. Whatever is knowable lies within our cognitive capacity. That is, the knowable is what our minds are capable of grasping and understanding. (Okay, there's more to it, but this is the bit that matters now.) On the other hand, there's faith. Faith relates to the limits of cognition, the boundaries around knowledge.
The idea is that your mind functions on the basis of certain rules. Whatever the rules do for your mind -- whatever they make it possible for you to conceive, for example -- that is knowable. But the rules themselves can't be known. They aren't knowable. This is because they say what the knowable is; they set the conditions on something being knowable. But whatever sets the conditions on something can't be that thing itself.
Not everything -able works like this. Reflecting, refracting or otherwise being affected by light is the condition (well, one of them) on something's being visible, and you can very well see that light is being reflected, refracted or what have you. But some do. But that's a physical process; most abstract -ables, like knowable, are such that the conditions lie outside what they are conditions on. Something is admirable because, among other things, it deserves praise. But that condition -- deserving praise -- is not itself admirable. It is the things that deserve praise that are admirable; the condition of deserving praise is not admirable at all.
Taking another example, something is imaginable, approximately, if an image of that something can be brought to mind. But that condition -- being an image of something which can be brought to mind -- is not itself imaginable. (Because it's not actually an image; it's a verbal definition.)
Knowable is like that, according to Kant, anyway. There is the condition on something's being knowable, and then there are the things that condition determines to be knowable. But the condition does not so determine itself.
So what is the condition, then? According to Kant, it is something thinkable, rather than knowable. It's something we can construct a thought for -- so, we can say it in a grammatical sentence. And, as something thinkable rather than knowable, we can take it on faith. According to Kant, this sort of faith is rational.
Applying this back to the religious case, which Kant actually does, we can say that god is a thought and a condition on the knowable. (It doesn't matter, but according to Kant, god is the condition on moral knowledge. Moral knowledge relates to happiness, and happiness to god.) So, you can think the thought of god and be rational to do so.