2 But, say they, those things which are in the mind, in that we can by the mind itself discern them, we have no need to know through the eyes of the body; but those things, which you say unto us that we should believe, you neither point to without, that through the eyes of the body we may know them; nor are they within, in our own mind, that by exercising thought we may see them. And these things they so say, as though any one would be bidden to believe, if that, which is believed, he could already see set before him.
Therefore certainly ought we to believe certain temporal things also, which we see not, that we may merit to see eternal things also, which we believe. But, whosoever you are who will not believe save what you see, lo, bodies that are present you see with the eyes of the body, wills and thoughts of your own that are present, because they are in your own mind, you see by the mind itself; tell me, I pray you, your friend's will towards you by what eyes do you see? For no will can be seen by the eyes of the body.
What? See you in your own mind this also which is going on in the mind of another? But if you see it not, how do you repay in turn the good will of your friend, if what you cannot see, you believe not? Will you haply say that you see the will of another through his works? Therefore you will see acts, and hear words, but concerning your friend's will, that which cannot be seen and heard you will believe. For that will is not color or figure, so as to be thrown upon the eyes; or sound or strain, so as to glide into the ears; nor indeed is it your own, so as to be perceived by the motion of your own heart.
It remains therefore that, being neither seen, nor heard, nor beheld within yourself, it be believed, that your life be not left deserted without any friendship, or affection bestowed upon you be not repaid by you in return. 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. Although a man may also deceive by feigning good will, and hiding malice: or, if he have no thought to do harm, yet by expecting some benefit from you, feigns, because he has not, love.
Source: Concerning Faith of Things Not Seen (New Advent)