4 John bare record because he saw. What record did he bear? “That this is the Son of God.” It behooved, then, that He should baptize who is God's only Son, not His adopted son. Adopted sons are the ministers of the only Son: the only Son has power; the adopted, the ministry. In the case that a minister baptizes who does not belong to the number of sons, because he lives evilly and acts evilly, what is our consolation? “This is He which baptizes.”
5. “The next day, John stood, and two of his disciples; and looking upon Jesus as He walked, he says, Behold the Lamb of God!” Assuredly, in a special sense, the Lamb; for the disciples were also called lambs: “Behold, I send you as lambs in the midst of wolves.” They were also called light: “You are the light of the world;” but in another sense is He called so, concerning whom it was said, “That was the true light, which lights every man that comes into the world.” In like manner was He called the dove in a special sense, alone without stain, without sin; not one whose sins have been washed away, but One who never had stain. For what? Because John said concerning the Lord, “Behold the Lamb of God,” was not John himself a lamb? Was he not a holy man? Was he not the friend of the Bridegroom? Wherefore, with a special meaning, said John of Him, “This is the Lamb of God;” because solely by the blood of this Lamb alone could men be redeemed.
Source: Tractates on the Gospel of John (New Advent)