11 You know that certain men say sometimes, We take away sin from men, we who are holy; for if he be not holy who baptizes, how takes he away the sin of another, when he is a man himself full of sin? In opposition to these disputations, let us not speak our own words, let us read what John says: “Behold the Lamb of God; behold Him who takes away the sin of the world!” Let there not be presumptuous confidence of men upon men: let not the sparrow flee to the mountains, but let it trust in the Lord; and if it lift its eyes to the mountains, from whence comes aid to it, let it understand that its aid is from the Lord who made heaven and earth. So great is the excellence of John, that to him it is said, “Are you the Christ?” He says, No. Are you Elias? He says, No. Are you a prophet? He says, No. Wherefore then do you baptize? “Behold the Lamb of God; behold Him who takes away the sin of the world! This is He of whom I spoke, After me comes a Man who was made before me; for He was before me.” “Comes after me,” because He was born later; “was made before me,” because preferred before me; “He was before me,” because, “In the beginning was the Word, and the Word was with God, and the Word was God.”
12. “And I knew Him not,” he said; “but that He might be made manifest to Israel, therefore came I baptizing with water. And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon Him. And 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, the same is He who baptizes with the Holy Ghost. And I saw, and bare record that this is the Son of God.” Give heed for a little, beloved. When did John learn Christ? For he was sent to baptize with water. They asked, Wherefore? That He might be made manifest to Israel, he said. Of what profit was the baptism of John? My brethren, if it had profited in any respect, it would have remained now, and men would have been baptized with the baptism of John, and thus have come to the baptism of Christ. But what says he? “That He might be made manifest to Israel,”— that is, to Israel itself, to the people Israel, so that Christ might be made manifest to it—therefore he came baptizing with water. John received the ministry of baptism, that by the water of repentance he might prepare the way for the Lord, not being himself the Lord; but where the Lord was known, it was superfluous to prepare for Him the way, for to those who knew Him He became Himself the way; therefore the baptism of John did not last long. But how was the Lord pointed out? Lowly, that John might so receive a baptism in which the Lord Himself should be baptized.
Source: Tractates on the Gospel of John (New Advent)