VII. Our Lord's baptism by John very different to the baptism of believers.
But if any one thinks the feast of the Epiphany, which in proper degree is certainly to be held in due honour, claims the privilege of baptism because, according to some the Lord came to St. John's baptism on the same day, let him know that the grace of that baptism and the reason of it were quite different, and is not on an equal footing with the power by which they are re-born of the Holy Ghost, of whom it is said, “which were born not of blood, nor of the will of the flesh, nor of the will of man, but of God.”
For the Lord who needed no remission of sin and sought not the remedy of being born again, desired to be baptized just as He desired to be circumcised, and to have a victim offered for His purification: that He, who had been “made of a woman,” as the Apostle says, might become also “under the law” which He had come, “not to destroy but to fulfil,” and by fulfilling to end, as the blessed Apostle proclaims, saying: “but Christ is the end of the law unto righteousness to every one that believes.”
But the sacrament of baptism He founded in His own person, because “in all things having the pre-eminence,” He taught that He Himself was the Beginning. And He ratified the power of re-birth on that occasion, when from His side flowed out the blood of ransom and the water of baptism. As, therefore, the Old Testament was the witness to the new, and “the law was given by Moses: but grace and truth came through Jesus Christ;” as the various sacrifices prefigured the one Victim, and the slaughter of many lambs was ended by the offering up of Him, of whom it is said, “Behold the Lamb of God; behold Him that takes away the sin of the world;” so too John, not Christ, but Christ's forerunner, not the bridegroom, but the friend of the bridegroom, was so faithful in seeking, “not His own, but the things which are Jesus Christ's,” as to profess himself unworthy to undo the shoes of His feet: seeing that He Himself indeed baptized “in water unto repentance,” but He who with twofold power should both restore life and destroy sins, was about to “baptize in the Holy Ghost and fire.”
As then, beloved brethren, all these distinct proofs come before you, whereby to the removal of all doubt you recognize that in baptizing the elect who, according to the Apostolic rule have to be purged by exorcisms, sanctified by fastings and instructed by frequent sermons, two seasons only are to be observed, viz. Easter and Whitsuntide: we charge you, brother, to make no further departure from the Apostolic institutions. Because hereafter no one who thinks the Apostolic rules can be set at defiance will go unpunished.
Source: Letters (New Advent)