Albert Barnes Commentary Matthew 3:15

"But Jesus answering said unto him, Suffer [it] now: for thus it becometh us to fulfil all righteousness. Then he suffereth him." — Matthew 3:15 (ASV)

Thus it becometh us (Matthew 3:15). It is fit and proper. And though you may feel yourself unworthy, it is still proper that it should be done.

All righteousness. There was no particular command in the Old Testament requiring this, but he chose to give the endorsement of his example to the baptism of John, as 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, show his connection with him, and give his approval to what John had done.

Also, he was baptized so that an opportunity might be taken, at the commencement of his work, for God to publicly declare his approval of him and his solemn appointment to the office of Messiah.