Thus it becometh us - It is suitable and proper. And though you may feel yourself unworthy, yet it is proper it should be done.
All righteousness - There was no particular precept in the Old Testament requiring this, but he chose to give the sanction of his example to the baptism of John, as to a divine ordinance. The phrase "all righteousness," here, is the same as a righteous institution or appointment. Jesus had no sin. But he was about to enter on his great work. It was proper that he should be set apart by his forerunner, and show his connection with him, and give his approbation to what John had done. He submitted to the ordinance of baptism, also, in order that occasion might be taken, at the commencement of his work, for God publicly to declare his approbation of him, and his solemn appointment to the office of the Messiah.

To fulfill all righteousness - That is, Every righteous ordinance: so I think the words πασαν δικαιοσυνην should be translated; and so our common version renders a similar word, Luke 1:6. The following passage, quoted from Justin Martyr, will doubtless appear a strong vindication of this translation. "Christ was circumcised, and observed all the other ordinances of the law of Moses, not with a view to his own justification; but to fulfill the dispensation committed to him by the Lord, the God and Creator of all things." - Wakefield.
How remarkable are the following words of Creeshna (an Incarnation of the Supreme God, according to the Hindoo theology) related in the Bhagvat Geeta, p. 47. Addressing his disciple Arjoon, he says, "I myself, Arjoon, have not, in the three regions of the universe, any thing which is necessary for me to perform; nor any thing to obtain, which is not obtained; and yet I live in the exercise of the moral duties. If I were not vigilantly to attend to those duties, all men would presently follow my example. If I were not to perform the moral actions, this world would fail in their duties: I should be the cause of spurious births, and should drive the people from the right way. As the ignorant perform the duties of life from a hope of reward, so the wise man, out of respect to the opinions and prejudices of mankind, should perform the same without motives of interest. The wise man, by industriously performing all the duties of life, should induce the vulgar to attend to them."
The Septuagint use this word often for the Hebrew משפת mishpat, judgment, appointment. And in Ezekiel 18:19, Ezekiel 18:21, the person who δικαιοσυνην και ελεος πεποιηκε - hath done righteousness and mercy, is he who sacredly attended to the performance of all the religious ordinances mentioned in that chapter, and performed them in the genuine spirit of mercy. Δικαιωματα is used 1 Maccabees 1:13, 49; 2:21, and in Hebrews 10:1, Hebrews 10:10, to denote religious ceremonies. Michaelis supposes that כל חק kol chok, all religious statutes or ordinances, were the words used in the Hebrew original of this Gospel.
But was this an ordinance? Undoubtedly: it was the initiatory ordinance of the Baptist's dispensation. Now, as Christ had submitted to circumcision, which was the initiatory ordinance of the Mosaic dispensation, it was necessary that he should submit to this, which was instituted by no less an authority, and was the introduction to his own dispensation of eternal mercy and truth. But it was necessary on another account: Our Lord represented the high priest, and was to be the high priest over the house of God: - now, as the high priest was initiated into his office by washing and anointing, so must Christ: and hence he was baptized, washed, and anointed by the Holy Ghost. Thus he fulfilled the righteous ordinance of his initiation into the office of high priest, and thus was prepared to make an atonement for the sins of mankind.
Then he suffered him - In the Opus Imperfectum, quoted by Griesbach, there is the following addition, which, at least, may serve to show the opinion of its author: Et Johannes quidem baptizauit ilium in aqua, ille autem Johannem cum Spiritu. "Then John baptized him with water, and he baptized John with the Spirit."

And Jesus answering said unto him, Suffer [it to be so] now: for thus it becometh us to fulfil (n) all righteousness. Then he suffered him.
(n) All such things as it has appointed for us to keep.

And Jesus answering, said unto him,.... This is an Hebrew way of speaking, often used in the Old Testament, and answers to see Job 3:1. He replied to John, who had made use of very forbidding words, after this manner,
suffer it to be so now; let me have my request; do not go on to object, but comply with my desire; let it be done now, immediately, directly, at this present time; do not put me off with any excuse; it is a proper season for it, even "now", since the time is not yet come that I am to baptize with the Holy Ghost; and besides, thus it becometh us to fulfil all righteousness. It became John to administer the ordinance of baptism to Christ, as he was his forerunner, and the only administrator of it, and that he might fulfil the ministry which he had received; and as it became Christ to fulfil all righteousness, moral and ceremonial, and baptism being a part of his Father's will, which he came to do, it became him to fulfil this also. And since it became Christ, it cannot be unbecoming us to submit to this ordinance; and since he looked upon it as a part of righteousness to be fulfilled by him, it ought to be attended to by all those who would be accounted followers of him. Christ having strongly urged the conveniency and equity of the administration of baptism to him, which showed his eager desire after it, and the lowliness of his mind; and John being convinced, and overcome by the force of his reasoning, agrees to his baptism;
then he suffered him, i.e. to be baptized in water by him, as he had requested, and accordingly did administer it to him.

And Jesus answering said unto him, Suffer it to be so now--"Let it pass for the present"; that is, "Thou recoilest, and no wonder, for the seeming incongruity is startling; but in the present case do as thou art bidden."
for thus it becometh us--"us," not in the sense of me and thee," or "men in general," but as in John 3:11.
to fulfil all righteousness--If this be rendered, with SCRIVENER, "every ordinance," or, with CAMPBELL, "every institution," the meaning is obvious enough; and the same sense is brought out by "all righteousness," or compliance with everything enjoined, baptism included. Indeed, if this be the meaning, our version perhaps best brings out the force of the opening word "Thus." But we incline to think that our Lord meant more than this. The import of circumcision and of baptism seems to be radically the same. And if our remarks on the circumcision of our Lord (see on Luke 2:21-24) are well founded, He would seem to have said, "Thus do I impledge Myself to the whole righteousness of the Law--thus symbolically do enter on and engage to fulfil it all." Let the thoughtful reader weigh this.
Then he suffered him--with true humility, yielding to higher authority than his own impressions of propriety.
Descent of the Spirit upon the Baptized Redeemer (Matthew 3:16-17).

Suffer it to be so now. The term "now" implies that the relation of Jesus to his work made it proper that now he should be baptized. It is true that baptism was for sinners; Jesus was sinless; but he humbled himself, accepted the burden of human duties, and must set a perfect example to men. He obeyed the Jewish law, and it was needful also that he obey the Divine rite that John had inaugurated.
Thus it becometh us. In order to fulfill all righteousness, show forth a perfect obedience, set a perfect example, it became him to submit to the institution of baptism, and it became John to administer it to him. "Us" refers to Jesus and John.

It becometh us to fulfil all righteousness - It becometh every messenger of God to observe all his righteous ordinances. But the particular meaning of our Lord seems to be, that it becometh us to do (me to receive baptism, and you to administer it) in order to fulfil, that is, that I may fully perform every part of the righteous law of God, and the commission he hath given me.