Our conception is framed according to this notion, when we believe that God was made man for us, as an example of humility, and to show the love of God towards us. For this it is which it is good for us to believe, and to retain firmly and unshakenly in our heart, that the humility by which God was born of a woman, and was led to death through contumelies so great by mortal men, is the chiefest remedy by which the swelling of our pride may be cured, and the profound mystery by which the bond of sin may be loosed. So also, because we know what omnipotence is, we believe concerning the omnipotent God in the power of His miracles and of His resurrection, and we frame conceptions respecting actions of this kind, according to the species and genera of things that are either ingrafted in us by nature, or gathered by experience, that our faith may not be feigned. For neither do we know the countenance of the Virgin Mary; from whom, untouched by a husband, nor tainted in the birth itself, He was wonderfully born. Neither have we seen what were the lineaments of the body of Lazarus; nor yet Bethany; nor the sepulchre, and that stone which He commanded to be removed when He raised Him from the dead; nor the new tomb cut out in the rock, whence He Himself arose; nor the Mount of Olives, from whence He ascended into heaven. And, in short, whoever of us have not seen these things, know not whether they are as we conceive them to be, nay judge them more probably not to be so. For when the aspect either of a place, or a man, or of any other body, which we happened to imagine before we saw it, turns out to be the same when it occurs to our sight as it was when it occurred to our mind, we are moved with no little wonder. So scarcely and hardly ever does it happen.
And yet we believe those things most steadfastly, because we imagine them according to a special and general notion, of which we are certain. For we believe our Lord Jesus Christ to be born of a virgin who was called Mary. But what a virgin is, or what it is to be born, and what is a proper name, we do not believe, but certainly know. And whether that was the countenance of Mary which occurred to the mind in speaking of those things or recollecting them, we neither know at all, nor believe. It is allowable, then, in this case to say without violation of the faith, perhaps she had such or such a countenance, perhaps she had not: but no one could say without violation of the Christian faith, that perhaps Christ was born of a virgin.
Wherefore, since we desire to understand the eternity, and equality, and unity of the Trinity, as much as is permitted us, but ought to believe before we understand; and since we must watch carefully, that our faith be not feigned; since we must have the fruition of the same Trinity, that we may live blessedly; but if we have believed anything false of it, our hope would be worthless, and our charity not pure: how then can we love, by believing, that Trinity which we do not know? Is it according to the special or general notion, according to which we love the Apostle Paul? In whose case, even if he was not of that countenance which occurs to us when we think of him (and this we do not know at all), yet we know what a man is. For not to go far away, this we are; and it is manifest he, too, was this, and that his soul joined to his body lived after the manner of mortals. Therefore we believe this of him, which we find in ourselves, according to the species or genus under which all human nature alike is comprised.
What then do we know, whether specially or generally, of that most excellent Trinity, as if there were many such trinities, some of which we had learned by experience, so that we may believe that Trinity, too, to have been such as they, through the rule of similitude, impressed upon us, whether a special or a general notion; and thus love also that thing which we believe and do not yet know, from the parity of the thing which we do know? But this certainly is not so.
Augustine's The Trinity, Book XIII 4.7-5.8.