14 But that you may know, my brethren, that not from a necessity of any chain of sin did the Lord come to this John, as the other evangelists say when the Lord came to him to be baptized, John himself said, “Comest Thou to me? I have need to be baptized by You.” What did He reply to him? “Suffer it to be so now: let all righteousness be fulfilled?” What means this, “let all righteousness be fulfilled”? I came to die for men, have I not to be baptized for men? What means “let all righteousness be fulfilled”?
Let all humility be fulfilled. What then? Was not He to accept baptism from a good servant who accepted suffering at the hands of evil servants? Give heed then. The Lord being baptized, if John for this end baptized, that by means of his baptism the Lord might manifest His humility, should no one else have been baptized with the baptism of John? But many were baptized with the baptism of John. When the Lord was baptized with the baptism of John, the baptism of John ceased. John was immediately cast into prison.
Afterwards we do not find that any one is baptized with that baptism. If, then, John came baptizing for this end, that the humility of the Lord might be made manifest to us, in order that we might not disdain to receive from the Lord that which the Lord had received from a servant, should John have baptized the Lord alone? But if John had baptized the Lord alone, some would have thought that the baptism of John was more holy than that of Christ: as if Christ alone had been found worthy to be baptized with the baptism of John, but the human race with that of Christ.
Give heed, beloved brethren. With the baptism of Christ we have been baptized, and not only we, but the whole world, and this will continue to the end. Which of us can in any respect be compared with Christ, whose shoe's latchet John declared himself unworthy to unloose? If, then, the Christ, a man of such excellence, a man who is God, had been alone baptized with the baptism of John, what were men likely to say? What a baptism was that of John! His was a great baptism, an ineffable sacrament; behold, Christ alone deserved to be baptized with the baptism of John.
And thus the baptism of the servant would appear greater than the baptism of the Lord. Others were also baptized with the baptism of John, that the baptism of John might not appear better than the baptism of Christ; but baptized also was the Lord, that through the Lord receiving the baptism of the servant, other servants might not disdain to receive the baptism of the Lord: for this end, then, was John sent.
Source: Tractates on the Gospel of John (New Advent)