5 And what says the Lord to him? “Suffer it to be so now, for thus it becomes us to fulfil all righteousness.” “Suffer it to be so now,” John; you are not wiser than I. You see as man; I foreknow as God. It becomes me to do this first, and thus to teach. I engage in nothing unbecoming, for I am invested with honour. Do you marvel, O John, that I am not come in my dignity? The purple robe of kings suits not one in private station, but military splendour suits a king: am I come to a prince, and not to a friend?
“Suffer it to be so now for thus it becomes us to fulfil all righteousness:”I am the Fulfiller of the law; I seek to leave nothing wanting to its whole fulfilment, that so after me Paul may exclaim, “Christ is the fulfilling of the law for righteousness to every one that believes.” “Suffer it to be so now, for thus it becomes us to fulfil all righteousness.” Baptize me, John, in order that no one may despise baptism. I am baptized by you, the servant, that no one among kings or dignitaries may scorn to be baptized by the hand of a poor priest.
Allow me to go down into the Jordan, in order that they may hear my Father's testimony, and recognise the power of the Son. “Suffer it to be so now, for thus it becomes us to fulfil all righteousness.” Then at length John suffers Him. “And Jesus, when He was baptized, went up straightway out of the water: and the heavens were opened unto Him; and, lo, the Spirit of God descended like a dove, and rested upon Him. And a voice (came) from heaven, saying, This is my beloved Son, in whom I am well pleased.”
Source: Discourse on the Holy Theophany (New Advent)