Matthew 15:9

In vain they do worship me, etc. - By the traditions of the elders, not only the word of God was perverted, but his worship also was greatly corrupted. But the Jews were not the only people who have acted thus: whole Christian Churches, as well as sects and parties, have acted in the same way. Men must not mould the worship of God according to their fancy - it is not what they think will do - is proper, innocent, etc., but what God himself has prescribed, that he will acknowledge as his worship. However sincere a man may be in a worship of his own invention, or of man's commandment, yet it profits him nothing. Christ himself says it is in vain. To condemn such, may appear to some illiberal; but whatever may be said in behalf of sincere heathens, and others who have not had the advantages of Divine Revelation, there is no excuse for the man who has the Bible before him.

In vain do they worship me - That is, their attempts to worship are "vain," or are not real worship - they are mere "forms."

Teaching for doctrines ... - The word "doctrines," here, means the requirements of religion - things to be believed and practiced in religion.

God only has a right to declare what shall be done in his service; but they held their traditions to be superior to the written word of God, and taught them as doctrines binding the conscience. See the notes at Isaiah 29:13.

15:9 Teaching the commandments of men - As equal with, nay, superior to, those of God. What can be a more heinous sin?