23 It is enough, then, that I have shown that there are some three things which are exhibited separately, whose operation is yet inseparable. If you have discovered this in your own self; if you have discovered it in man; if you have discovered it in a being that walks on the earth, and bears about a frail “body, which weighs down the soul;” believe that the Father, Son, and Holy Spirit may be exhibited separately, by certain visible symbols, by certain forms borrowed from the creatures, and still their operation be inseparable.
This is enough. I do not say that “memory” is the Father—the “understanding” the Son—and “will” the Spirit; I do not say this; let men understand it how they will. I do not venture to say this. Let us reserve the greater truths for those who are capable of them: but, infirm as I am myself, I convey to the infirm only what is according to our powers. I do not say that these things are in any sort to be equalled with the Holy Trinity, to be squared after an analogy; that is, a kind of exact rule of comparison.
This I do not say. But what do I say? See. I have discovered in you three things, which are exhibited separately, whose operation is inseparable; and of these three, every single name is produced by the three together; yet does not this name belong to the three, but to some one of the three. Believe then in the Trinity, what you can not see, if in yourself you have heard, and seen, and retained it. For what is in your own self you can know: but what is in Him who made you, whatever it be, how can you know?
And if you shall be ever able, you are not able yet. And even when you shall be able, will you be able so to know God, as He knows Himself? Let then this suffice you, beloved: I have said all I could; I have made good my promise as you required. As to the rest which must be added, that your understanding may make advancement, this seek from the Lord.
Source: Sermons on Selected Lessons of the New Testament (New Advent)