With the coming of Jesus to enter upon His work, it might have been supposed that the mission of John the Baptist would cease, its end being accomplished. However, his ministry did not wholly cease, since he had not brought the nation to repentance; but it changed its form. His baptism could no more have a general and indefinite reference to one still to come (see Acts 4:4). Having declared Jesus of Nazareth to be the Messiah, the undefined Messianic hopes of the nation were now to be concentrated upon Him. All the teachings and labors of the Baptist pointed to Him, and all tended to prepare the people to receive Him. Whether there was any change in the baptismal formula may be doubted, but the immediate and personal reference to Jesus as the Messiah was that which distinctively characterized the last stage of John's work, and explains why his baptism still continued.
But why should John continue to baptize? It need not be said that if the rulers and people had responded to his preaching of repentance, and thus been prepared to receive the Lord, he would not have continued this work. But it was an indispensable condition to the reception of the Christ, the Holy One of God, that sin should be repented of and put away. Upon this John had insisted in his preaching: "Repent, for the kingdom of God is at hand." But this preaching and this baptism, both pointing to repentance, were no less important now that the Messiah had actually come. Without holiness of heart they could not receive Him, could not even discern Him as the Messiah. John had already baptized many into the hope of His coming, but others had equal need to be baptized into the reality of it.
We can now see why John should have continued baptizing after the Lord came and why Jesus should Himself, through His disciples, also baptized. It was not enough that He had personally come. All those that, with hearts conscious of guilt, both personal and national, and truly penitent, were "waiting for the consolation of Israel," were willing to be baptized, confessing their sins; but the unrepentant, the unbelieving, the self-righteous, all who justified themselves, rejected the rite (Luke 7:29-30).
Adapted from The Life of Our Lord upon the Earth by Samuel James Andrews.