Albert Barnes 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, and I will testify to you ... - See the notes at the similar passage in Psalm 50:7. God calls their attention to what he required of them; to what his law demanded; to what was the condition of their being his people and of securing his favor. What he demanded was, that they should acknowledge him; obey him; serve him; that there should be no strange god among them, and that they should worship no false god (Psalms 81:9).