1 There are those who think that the Christian religion is what we should smile at rather than hold fast, for this reason, that, in it, not what may be seen, is shown, but men are commanded faith of things which are not seen. We therefore, that we may refute these, who seem to themselves through prudence to be unwilling to believe what they cannot see, although we are not able to show unto human sight those divine things which we believe, yet do show unto human minds that even those things which are not seen are to be believed.
And first they are to be admonished, (whom folly has so made subject to their carnal eyes, as that, whatsoever they see not through them, they think not that they are to believe,) how many things they not only believe but also know, which cannot be seen by such eyes. Which things being without number in our mind itself, (the nature of which mind is incapable of being seen,) not to mention others, the very faith whereby we believe, or the thought whereby we know that we either believe any thing, or believe not, being as it is altogether alien from the sight of those eyes; what so naked, so clear, what so certain is there to the inner eyes of our minds?
How then are we not to believe what we see not with the eyes of the body, whereas, either that we believe, or that we believe not, in a case where we cannot apply the eyes of the body, we without any doubt see?
Source: Concerning Faith of Things Not Seen (New Advent)