John Calvin Commentary Psalms 81:8

"Hear, O my people, and I will testify unto thee: O Israel, if thou wouldest hearken unto me!" — Psalms 81:8 (ASV)

Hear, O my people! To touch the hearts of the people more effectively, God is here presented with the character of a teacher, and introduced as speaking familiarly among the congregation. This is done to instruct them that all assemblies are fruitless and trivial in which the voice of God, stirring up people to faith and true godliness, is not heard.

But let us proceed to consider the words. This preface was intended to teach, in a few words, that festival days were not truly and rightly observed unless the people listened attentively to the voice of God. To consecrate their hands, feet, eyes, and their whole persons to His service, it was necessary for them, first, to open their ears to His voice.

Thus, the lesson is taught that He acknowledges as His servants only those who are willing to learn. By the word protest, He indicates that He makes a solemn covenant, thus giving His words greater authority. The clause that follows, O Israel! if you will listen to me, is, I presume, an abrupt expression, similar to what is frequently used in impassioned discourses, the ellipsis serving to express greater earnestness.

Some connect it with the following verse in this way: O Israel! if you will listen to me, there will be no strange god in you. But it is rather to be viewed as the language of regret on God’s part. He indirectly suggests that He distrusts this obstinate and rebellious people, and can hardly entertain the hope that they will be obedient and teachable.