3 But you say, that you therefore believe your friend, whose heart you cannot see, because you have proved him in your trials, and have come to know of what manner of spirit he was towards you in your dangers, wherein he deserted you not. Seems it therefore to you that we must wish for our own affliction, that our friend's love towards us may be proved? And shall no man be happy in most sure friends, unless he shall be unhappy through adversity? So that, forsooth, he enjoy not the tried love of the other, unless he be racked by pain and fear of his own?
And how in the having of true friends can that happiness be wished for, and not rather feared, which nothing save unhappiness can put to the proof? And yet it is true that a friend may be had also in prosperity, but proved more surely in adversity. But assuredly in order to prove him, neither would you commit yourself to dangers of your own, unless you believed; and thus, when you commit yourself in order to prove, you believe before you prove. For surely, if we ought not to believe things not seen, since indeed we believe the hearts of our friends, and that, not yet surely proved; and, after we shall have proved them good by our own ills, even then we believe rather than see their good will towards us: except that so great is faith, that, not unsuitably, we judge that we see, with certain eyes of it, that which we believe, whereas we ought therefore to believe, because we cannot see.
Source: Concerning Faith of Things Not Seen (New Advent)