Jamieson, Fausset and Brown on Ephesians 6:2

2. Here the authority of revealed law is added to that of natural law.

which is ... promise--The "promise" is not made the main motive to obedience, but an incidental one. The main motive is, because it is God's will (De 5:16, "Honor thy father and mother, as the Lord thy God hath COMMANDED thee"); and that it is so peculiarly, is shown by His accompanying it "with a promise."

first--in the decalogue with a special promise. The promise in the second commandment is a general one. Their duty is more expressly prescribed to children than to parents; for love descends rather than ascends [Bengel]. This verse proves the law in the Old Testament is not abolished.