Those who question the values of Old Testament study should consider the fact that no place in the word of God defines obedience with greater precision and clarity than the book of Deuteronomy. Special attention should be focused upon three passages:
"Ye shall not add unto the word which I command you, neither shall ye diminish from it, that ye may keep the commandments of Jehovah your God which I command you" (Deut. 4:2).
"Ye shall observe to do therefore as Jehovah your God hath commanded you: ye shall not turn aside to the right hand or to the left." (Deut. 5:32).
"What thing soever I command you, that shall ye observe to do: thou shalt not add thereto, nor diminish from it" (Deut. 12:32).
What is suggested is that obedience requires doing exactly what God commands without modification. Israel was warned against adding to or diminishing from God's commandment, and in the first of the three verses the purpose of such strict adherence to the divine command is assigned: "that ( i.e., in order that) ye may keep the commandments of Jehovah your God." This is simply saying that the only way to keep the commandments of God is to do exactly as God says, without addition or subtraction.
When one adds to or diminishes from the divine commandment, and then fulfills the modified version, he has not kept the commandment of God at all; he has done as he pleases. That is a lesson worth remembering. The commandments of God that are in force today are not the same commandments which God gave in an earlier age. But the meaning of obedience does not change, no matter what commandments God gives.
- L.A. Mott, Jr. via Searching The Scriptures, Feb., 1973