John the Baptist has a point when he objects, “I need to be baptized by you, and do you come to me?” (Matthew 3:14). But there’s more going on here than he first realizes.
Here’s Don Carson on what Jesus means in Matthew 3:15 in responding to John the Baptist, “Let it be so now, for thus it is filling for us to fulfill all righteousness”—
Jesus affirms, in effect, that it is God’s will (“all righteousness”) that John baptize him; and both John and Jesus “fulfill” that will, that righteousness, by going through with it (“it is proper for us”). The aftermath, as Matthew immediately notes (vv. 16–17), shows that this baptism really did point to Jesus.
Within this framework we may recognize other themes. In particular Jesus is indeed seen as the Suffering Servant (Isa 42:1). But the Servant’s first mark is obeying God: he “fulfills all righteousness” since he suffers and died to accomplish redemption in obedience to the will of God. By his baptism Jesus affirms his determination to do his assigned work.
Thus the “now” may be significant: Jesus is saying that John’s objection (v. 14) is in principle valid. Yet he must “now,” at this point in salvation history, baptize Jesus for at this point Jesus must demonstrate his willingness to take on his servant role, entailing his identification with the people (Matthew, 108, paragraphing added).