13 Then cometh Jesus from Galilee to the Jordan, unto John, to be baptized by him.
Pseudo-Chrysostom: “Then,” that is when John preached, that He might confirm his preaching, and Himself receive his witness. But as when the morning-star has risen, the sun does not wait for that star to set, but rising as it goes forward, gradually obscures its brightness; so Christ waited not for John to finish his course, but appeared while he yet taught.
Ambrose, Ambrosiaster: Scripture tells of many wonders wrought at various times in this river; as that, among others, in the Psalms, “Jordan was driven backwards;” [Ps 114:3] before the water was driven back, now sins are turned back in its current; as Elijah divided the waters of old, so Christ the Lord wrought in the same Jordan the separation of sin.
Remigius: Not baptism to the remission of sins, but to leave the water sanctified for those after to be baptized.
Augustine: The Saviour willed to be baptized not that He might Himself be cleansed, but to cleanse the water for us.
Pseudo-Chrysostom: He comes to baptism, that He who has taken upon Him human nature, may be found to have fulfilled the whole mystery of that nature; not that He is Himself a sinner, but He has taken on Him a nature that is sinful. And therefore though He needed not baptism Himself, yet the carnal nature in others needed it.
Ambrose, Ambrosiaster: Also like a wise master inculcating His doctrines as much by His own practice, as by word of mouth, He did that which He commanded all His disciples to do.
Augustine: He deigned to be baptized of John that the servants might see with what readiness they ought to run to the baptism of the Lord, when He did not refuse to be baptized of His servant.
Jerome: That by being Himself baptized, He might sanction the baptism of John.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).