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.”
Source: Tractates on the Gospel of John (New Advent)