Albert Barnes Commentary Psalms 81:4

"For it is a statute for Israel, An ordinance of the God of Jacob." — Psalms 81:4 (ASV)

For this was a statute for Israel... - . That is, it was a law for the whole Jewish people, for all who had the name Israel, for all the descendants of Jacob. The word "was" is not in the original, as if this had been an old commandment that might now be obsolete; but the idea is one of perpetuity: it is a perpetual law for the Hebrew people.

A law of the God of Jacob - Hebrew, a judgment, or right. The idea is that it was what was due to God, what was His right. It was a solemn claim that He should be thus acknowledged. It was not a matter of conventional arrangement or convenience to them, nor was it to be observed merely because it was found to be expedient and conducive to the welfare of the nation. It was a matter of right and of claim on the part of God, and was so to be regarded by the nation.

The same is true now of the Sabbath and of all the appointments God has made for maintaining religion in the world. All these arrangements are indeed expedient and proper; they contribute to the public welfare and to the happiness of humanity, but there is a higher reason for their observance than this.

It is that God demands their observance, that He claims as His own the time so appropriated. Thus He claims the Sabbath, the entire Sabbath, as His own; He requires that it shall be employed in His service, that it shall be regarded as His day, that it shall be made instrumental in maintaining the knowledge of Himself in the world, and in promoting His glory (Exodus 20:10). People, therefore, rob God when they take this time for needless secular purposes or devote it to other ends and uses. Nor can this be sinless. The highest guilt a person can commit is to rob their Maker of what belongs to Him and of what He claims.