A.T. Robertson Commentary Matthew 5:19

"Whosoever therefore shall break one of these least commandments, and shall teach men so, shall be called least in the kingdom of heaven: but whosoever shall do and teach them, he shall be called great in the kingdom of heaven." — Matthew 5:19 (ASV)

Shall do and teach (ποιηση κα διδαξη). Jesus puts practice before preaching. The teacher must apply the doctrine to himself before he is qualified to teach others. The scribes and Pharisees were men who "say and do not" (Matthew 23:3), who preach but do not perform. This is Christ's test of greatness.