John Calvin on Deuteronomy 32:12

12. So the Lord alone did lead hive. This is spoken by anticipation, in order to take away every pretext from the Israelites, provided they should seek, according to their custom, to mingle their superstitions with the pure service of God. For, when they were bringing in, from all quarters, gods of various nations, this was the excuse they commonly made, that God was not thus despoiled of His due honor: and hence it came to pass, that they permitted themselves to heap together a multitude of false gods, whom they worshipped as their patrons. But Moses anticipates them, and declares that God, as having no need of external aid, had not associated with Himself any strange gods in His preservation of the people. Hence it follows, that whatever gods the people introduced, they transferred to them the honor due to the one true God. Let us then learn from this passage, that, unless God be served without a rival, religion is altogether perverted by the impious admixture.