2 Did John hear this that he might know Him whom he had not known, or that he might more fully know Him whom he had already known? For if he had been entirely ignorant of Him, he would not have said to Him when He came to the river to be baptized, “I have need to be baptized by You, and You come to me?” He knew Him therefore. But when did the dove descend? When the Lord had been baptized, and was ascending from the water. But if He who sent Him said, “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He which baptizes with the Holy Ghost,” and he knew Him not, but when the dove descended he learned to know Him, and the time at which the dove descended was when the Lord was going up from the water; but John had known the Lord, when the Lord came to him to the water: it is made plain to us that John after a manner knew, and after a manner did not at first know the Lord.
And unless we understand it so, he was a liar. How was he true acknowledging the Lord and saying, “Comest Thou to me to be baptized,” and, “I have need to be baptized by You”? Is he true when he said this? And how is he again true when he says, “I knew Him not: 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 who baptizes with the Holy Ghost”? The Lord was made known by a dove, not to him who knew Him not, but to him who in a manner knew Him, and in a manner knew Him not. It is for us to discover what, in Him, John did not know, and learned by the dove.
Source: Tractates on the Gospel of John (New Advent)