John Calvin Commentary Psalms 81:9

"There shall no strange god be in thee; Neither shalt thou worship any foreign god." — Psalms 81:9 (ASV)

Let there be no strange god in thee. Here the leading article of the covenant is set forth, and almost the whole sum of it: that God alone must have the preeminence. Some may prefer this explanation: O Israel! If you will listen to me, there is nothing which I more strictly require or demand from you than that you should be content with me alone, and that you should not seek after strange gods; and of this opinion I am far from disapproving.

God by this language undoubtedly confirms the truth which he so frequently teaches elsewhere in the Law and the Prophets: that he is so jealous a God as not to allow another to be a sharer of the honor to which he alone is entitled. But at the same time, he teaches us that true religious worship begins with obedience.

The order which Moses observes is different (Exodus 20:2, 4, and Deuteronomy 5:6, 8). In these passages, God begins by declaring that he is the God of Israel, and then he forbids them to make for themselves any new gods. But here the prohibition is put first, and then the reason for it is added: that the people should be abundantly satisfied with the God who had purchased them to be his people. Perhaps also he sets this first to prepare the way for his obtaining the throne of their hearts. He would first withdraw the people from superstitions, as these must necessarily be plucked up and cleared away before true religion can take root in our hearts.