8 But this John did not know in the Lord. That He was the Lord he knew, and that he ought to be baptized by Him he knew; and he confessed that He was the Truth, and that he, the true man, was sent by the Truth: this he knew. But what was in Him which he knew not? That he was about to retain to Himself the power of His baptism, and was not to transmit or transfer it to any servant; but that, whether a good servant baptized in a ministerial manner, or whether an evil servant baptized, the person baptized should not know that he was baptized, unless by Him who kept to Himself the power of baptizing.
And that you may know, brethren, what John did not know in Him, he learned it by means of the dove: for he knew the Lord; but that He was to retain to Himself the power of baptizing, and not to give it to any servant, he did not yet know. Regarding this he said, “I knew Him not.” And that you may know that he there learned this, give heed to what follows: “But He that sent me to baptize with water, the same said unto me, Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He.”
What same is He? The Lord? But he already knew the Lord. Suppose, then, that John had said thus far, “I knew Him not: but He that sent me to baptize with water, the same said unto me—” We ask, what He said? It follows: “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him.” I do not say what follows. In the meantime give heed: “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He.” But what same is He? What did He who sent me mean to teach me by means of a dove?
That He was Himself the Lord. Already I knew by whom I was sent; already I knew Him to whom I said, “Comest Thou to me to be baptized? I have need to be baptized by You.” So far, then, did I know the Lord, that I wished to be baptized by Him, not that He should be baptized by me; and then He said to me, “Suffer it to be so now; for thus it becomes us to fulfill all righteousness.” I came to suffer; do I not come to be baptized? “Let all righteousness be fulfilled,” says my God to me.
Let all righteousness be fulfilled; let me teach entire humility. I know that there will be proud ones in my future people; I know that some men then will be eminent in some grace, so that when they see ordinary persons baptized, they, because they consider themselves better, whether in continence, or in almsgiving, or in doctrine, will perhaps not deign to receive what has been received by their inferiors. It was needful that I should heal them, so that they should not disdain to come to the baptism of the Lord, because I came to the baptism of the servant.
Source: Tractates on the Gospel of John (New Advent)