Jamieson, Fausset and Brown on Matthew 3:15

15. 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 Joh 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 Lu 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 (Mt 3:16, 17).