Matthew 3:15

For by Him must all righteousness have been fulfilled, by whom alone the Law could be fulfilled.

(Ambrosiaster. Serm. xii. 1.) 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.

To prove that this was to Him the last good work of those enjoined by the law, hear His own words: "For thus it becometh us to fulfill all righteousness." Now what He saith is like this: "We have performed all the duties of the law, we have not transgressed so much as one commandment. Since therefore this only remains, this too must be added, and so shall we 'fulfill all righteousness.'" For He here calls by the name of "righteousness" the full performance of all the commandments.

And he said not, "And art Thou baptized of me?" nay, for this he feared to say: but what? "And comest Thou to me?" What then doth Christ? What He did afterwards with respect to Peter, this did He then also. For so he too would have forbidden Him to wash his feet, but when he had heard, "What I do thou knowest not now, but thou shalt know hereafter," and "thou hast no part with me," he speedily withdrew from his determination, and went over to the contrary. And this man again in like manner, when he had heard, "Suffer it to be so now, for thus it becometh us to fulfill all righteousness," straightway obeyed. For they were not unduly contentious, but they manifested both love and obedience, and made it their study to be ruled by their Lord in all things. And mark how He urges him on that very ground which chiefly caused him to look doubtfully on what was taking place; in that He did not say, "thus it is just," but "thus it becometh." For, inasmuch as the point unworthy of Him was in his mind chiefly this, His being baptized by His servant, He stated this rather than anything else, which is directly opposed to that impression: as though He had said, "Is it not as unbecoming that thou avoidest and forbiddest this? nay, for this self-same cause I bid thee suffer it, that it is becoming, and that in the highest degree." And He did not merely say, "suffer," but He added, "now." "For it will not be so forever," saith He, "but thou shalt see me such as thou desirest; for the present, however, endure this." Next He shows also how this "becometh" Him. How then doth it so? "In that we fulfill the whole law;" and to express this He said, "all righteousness." For righteousness is the fulfilling of the commandments. "Since then we have performed all the rest of the commandments," saith He, "and this alone remains, it also must be added: because I am come to do away the curse that is appointed for the transgression of the law. I must therefore first fulfill it all, and having delivered you from its condemnation, in this way bring it to an end. It becometh me therefore to fulfill the whole law, by the same rule that it becometh me to do away the curse that is written against you in the law: this being the very purpose of my assuming flesh, and coming hither."

Beautifully said is that now, to show that as Christ was baptized with water by John, so John must be baptized by Christ with the Spirit. Or, suffer now that I who have taken the form of a servant should fulfil all that low estate; otherwise know that in the day of judgment thou must be baptized with my baptism. Or, the Lord says, 'Suffer this now; I have also another baptism wherewithal I must be baptized; thou baptizest Me with water, that I may baptize thee for Me with thy own blood.' Righteousness; but he adds neither 'of the Law;' nor 'of nature,' that we may understand it of both.