Ye shall {b} not add unto the word which I command you, neither shall ye {c} diminish ought from it, that ye may keep the commandments of the LORD your God which I command you.

(b) Think not to be more wise than I am.

(c) God will not be served by halves, but will have full obedience.

Jamieson-Fausset-Brown Bible Commentary

2. Ye shall not add unto the word which I command you-by the introduction of any heathen superstition or forms of worship different from those which I have appointed (De 12:32; Nu 15:39; Mt 15:9).

neither shall ye diminish aught from it-by the neglect or omission of any of the observances, however trivial or irksome, which I have prescribed. The character and provisions of the ancient dispensation were adapted with divine wisdom to the instruction of that infant state of the church. But it was only a temporary economy; and although God here authorizes Moses to command that all its institutions should be honored with unfailing observance, this did not prevent Him from commissioning other prophets to alter or abrogate them when the end of that dispensation was attained.

Matthew Henry's Concise Commentary

4:1-23 The power and love of God to Israel are here made the ground and reason of a number of cautions and serious warnings; and although there is much reference to their national covenant, yet all may be applied to those who live under the gospel. What are laws made for but to be observed and obeyed? Our obedience as individuals cannot merit salvation; but it is the only evidence that we are partakers of the gift of God, which is eternal life through Jesus Christ, Considering how many temptations we are compassed with, and what corrupt desires we have in our bosoms, we have great need to keep our hearts with all diligence. Those cannot walk aright, who walk carelessly. Moses charges particularly to take heed of the sin of idolatry. He shows how weak the temptation would be to those who thought aright; for these pretended gods, the sun, moon, and stars, were only blessings which the Lord their God had imparted to all nations. It is absurd to worship them; shall we serve those that were made to serve us? Take heed lest ye forget the covenant of the Lord your God. We must take heed lest at any time we forget our religion. Care, caution, and watchfulness, are helps against a bad memory.