5 For this purpose therefore did He receive baptism from John, in order that, receiving what was inferior from an inferior, He might exhort inferiors to receive that which was superior. But wherefore was not He alone baptized by John, if John, by whom Christ was baptized, was sent for this end, to prepare a way for the Lord, that is, for Christ Himself? This we have already explained, but we recur to it, because it is necessary for the present question. If our Lord Jesus Christ had been alone baptized with the baptism of John;— hold fast what we say; let not the world have such power as to efface from your hearts what the Spirit of God has written there; let not the thorns of care have such power as to choke the seed which is being sown in you: for why are we compelled to repeat the same things, but because we are not sure of the memory of your hearts?— and if then the Lord alone had been baptized with the baptism of John, there would be persons who would so reckon it, that the baptism of John was greater than is the baptism of Christ.
For they would say, that baptism is so much the greater, that Christ alone deserved to be baptized with it. Therefore, that an example of humility might be given us by the Lord, that the salvation of baptism might be obtained by us, Christ accepted what for Him was not necessary, but on our account was necessary. And again, lest that which Christ received from John should be preferred to the baptism of Christ, others also were permitted to be baptized by John. But for those who were baptized by John that baptism did not suffice: for they were baptized with the baptism of Christ; because the baptism of John was not the baptism of Christ.
Those who receive the baptism of Christ do not seek the baptism of John; those who received the baptism of John sought the baptism of Christ. Therefore was the baptism of John sufficient for Christ. How should it not be sufficient, when not even it was necessary? For to Him was no baptism necessary; but in order to exhort us to receive His baptism, He received the baptism of His servant. And lest the baptism of the servant should be preferred to the baptism of the Lord, other fellow-servants were baptized with the baptism of the servant. But it behooved those fellow-servants who were baptized with that baptism to be likewise baptized with the baptism of the Lord: but those who were baptized with the baptism of the Lord do not require the baptism of the fellow-servant.
Source: Tractates on the Gospel of John (New Advent)