Augustine of Hippo (A.D. 354 - 430), a.k.a. Saint Augustine, was an accomplished philosopher and theologian, and showed remarkable insight in his philosophical writings. The preceding sentence is an exercise in understatement. How much of an understatement? Consider that there is a compelling case to be made that René Descartes' "I think, therefore I am," which helped launch modern European philosophy, was an uncredited borrowing of an idea found in Augustine's Enchiridion.
Another line of thought that can be found in Augustine and has been picked up again in recent Anglo-American philosophy is the importance of belief in other minds for epistemology. Why is it important? Because a good epistemology ought to explain how it is that we know the things that we do know, and so if an epistemology implies that we don't actually know something that it seems plain that we do know, then that is a strike against that epistemology (I'm running rough-shod over the problem of the criterion here, but this is a blog post, and brevity is the soul of blogging wit, so...). Many philosophers have cited belief in other minds--for example, my belief that these other human bodies I see walking around contain conscious selves who experience vivid awareness of the world, feel emotions, and exercise rational thought just as I do--as a kind of belief that will trouble some epistemologies. (See, for example, J. L. Austin's 1946 essay "Other Minds," and Alvin Plantinga's 1967 monograph God and Other Minds.) Well, it turns out that Augustine cited just those sorts of beliefs for just that sort of purpose in his Concerning Faith of Things Not Seen. Consider the following passage:
Where then is that which you said, that you ought not to believe, save what you saw either outwardly in the body, or inwardly in the heart? Lo, out of your own heart, you believe an heart not your own; and lendest your faith, where you do not direct the glance of your body or of your mind. Your friend's face you discern by your own body, your own faith you discern by your own mind; but your friend's faith is not loved by you, unless there be in you in return that faith, whereby you may believe that which in him you see not.
Here Augustine is addressing someone who apparently embraces a kind of fairly strict empiricism, saying that we ought believe only what we perceive through our senses ("what you saw... outwardly in the body") or by introspection ("what you saw... inwardly in the heart"). Augustine accuses those who hold such a view of displaying a kind of epistemological hypocrisy: they believe that they have friends, in spite of not being able to see friendship (i.e., "your friend's faith"). They see their friends' bodies with their eyes, and they sense their own affection and friendliness toward their friends by introspection; but they do not see their friends' reciprocation of friendship in either way. Nevertheless, they do believe that their friends really are their friends. So they do have faith in that which they do not see, namely, an attitude of friendship in the other person. To put this in contemporary terms, they believe in certain mental states in others that are not evident through what is available to the senses or introspection.
Certain replies could be made. Contemporary theories of "mindreading", or social cognition, do include those that try to base knowledge of others' mental states entirely on sense perception and introspection (I would put both "theory theory" and "simulation theory" in this camp), and they have ways of responding to objections such as those raised by Augustine. I personally do not find those responses very convincing; but the point here is that Augustine saw the issue, and recognized its epistemological significance, over 1500 years ago.
Of course, Augustine's primary concern is religious, and with countering those who accuse Christians of irrationality because they believe in things that cannot be seen. This seems to be how Augustine is using the term "faith": believing in what one does not or even cannot see, including the "seeing" involved in introspection and rational insight. (Incidentally, C. S. Lewis uses the term in the same way--faith is opposed to sight, he says, not opposed to reason.) In the passage above, Augustine is using a defensive strategy commonly used even today by philosophers in various areas: show that the standard by which an opponent judges your view is a standard so high that even they cannot meet it. If belief on faith is, in principle, irrational and to be avoided, then we have to toss out belief in other minds as well. Or so says Augustine.
Another interesting feature in this particular work is that Augustine also brings in testimonial belief as an example of belief that is "faith in things not seen." Consider this passage:
I omit to mention in how many things they, who find fault with us because we believe what we see not, believe report or history; or concerning places where they have not themselves been; and say not, we believe not, because we have not seen. Since if they say this, they are obliged to confess that their own parents are not surely known to them: because on this point also they have believed the accounts of others telling of it, who yet are unable to show it, because it is a thing already past; retaining themselves no sense of that time, and yet yielding assent without any doubting to others speaking of that time: and unless this be done, there must of necessity be incurred a faithless impiety towards parents, while we are, as it were, showing a rashness of belief in those things which we cannot see.
Augustine's point here is that contingent events in history (what Hume would later call "matters of fact and existence") that we did not personally experience, or cannot personally remember, but believe to have occurred on the basis of what others tell us, belong as much to the realm of "faith" as does belief in other minds. David Hume would later (in the early Modern period) try to ground the rationality of testimonial belief on inductive reasoning from sensory experience, and so-called "reductionists" about testimonial knowledge (or justification) follow Hume in this; but such a view is increasingly challenged in the epistemology of testimony today. Many of us epistemologists are now agreeing with Augustine: the rationality of believing testimony cannot be reduced to the rationality of sensory experience plus inductive inference.
We cannot help but believe in things we cannot see, and so, in this sense, we cannot help but have faith. If we cannot have faith in other minds or in testimony, or if we try to live without faith, untoward consequences follow. So if there is something wrong with Christianity, Augustine argues, it cannot be that it requires faith. That kind of in-principle, philosophical, epistemological objection is too strong. That bomb will blow up the person who tries to use it. So if Christianity is bad religion, it must be shown to be so on other grounds; the faith requirement is not a reason to throw it out.
Of course, this is faith understood in a certain way, not the kind of faith that is "believing without any evidence," or even worse, "believing against all the evidence," or at the worst, what Mark Twain called "believing what you know ain't so." Augustine goes on in this work to talk about the kinds of evidences that are available. But those evidences support faith, rather than replace it. Belief in what cannot be seen, though well rationally supported, remains faith-based belief.