15 But did he know Christ, or did he not know Him? If he did not know Him, wherefore did He say, when Christ came to the river, “I have need to be baptized by You”? That is to say, I know who You are. If, then, he already knew Him, assuredly he knew Him when he saw the dove descending. It is evident that the dove did not descend upon the Lord until after He went up out of the water of baptism. “The Lord having been baptized, went up out of the water, and the heavens were opened, and he saw a dove descending on Him.”
If, then, the dove descended after the baptism, and if, before the Lord was baptized, John said to Him, “Comest Thou to me? I have need to be baptized by You;” that is to say, before he knew Him to whom he said, “Comest Thou to me? I have need to be baptized by You;”— how then said he, “And I knew Him not: but He who sent me to baptize with water, the same said to me, Upon whom you see the Spirit descending as a dove, and abiding upon Him, the same is He which baptizes with the Holy Ghost?”
It is not an insignificant question, my brethren. If you have seen the question, you have seen not a little; it remains that the Lord give the solution of it. This, however, I say, if you have seen the question, it is no small matter. Behold, John is placed before your eyes, standing beside the river. Behold John the Baptist. Behold, the Lord comes, as yet to be baptized, not yet baptized. Hear the voice of John, “Comest Thou to me? I have need to be baptized by You.” Behold, already he knew the Lord, by whom He wishes to be baptized.
The Lord, having been baptized, goes up out of the water; the heavens are opened, the Spirit descends; then John knows Him. If then for the first time he knew Him, why did he say before, “I have need to be baptized by You”? But if he did not then recognize Him for the first time, because he knew Him already, what is the meaning of what he said, “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, and abiding upon Him, as a dove, the same is He which baptizes with the Holy Ghost”?
Source: Tractates on the Gospel of John (New Advent)