Deuteronomy 32:21
They have moved me to jealousy with that which is not God; they have provoked me to anger with their vanities: and I will move them to jealousy with those which are not a people; I will provoke them to anger with a foolish nation. They have moved me to jealousy with that which is not God; they have provoked me to anger with their vanities: and I will move them to jealousy with those which are not a people; I will provoke them to anger with a foolish nation.
Israel stirred divine jealousy by turning to what was not God and stirred divine anger by turning to empty, vain, worthless idols and false gods, and in return God would stir Israel to jealousy by those who are not a people and stir Israel to anger by a foolish nation. That retaliatory people was one Israel itself regarded as no people, not truly a people, and that retaliatory nation was one Israel regarded as foolish, insignificant and of no account. The warning was that unfaithfulness and departure from God would be answered in kind and would bring the judgments that follow such a course.
This declaration came after God abhorred His sons and daughters because of their provoking, hid His face from them to see what their end would be, and called them a perverse and forward generation and children with no faith.
It belongs to Moses' song and account that recounts God's past dealings with His people and their departure from Him. Moses was commanded to teach that song to Israel so as to preserve the record of those dealings for them and their children. That song foreshadows future events and judgments, including the final victory of the faithful when Christ comes again in power and glory.
In its setting among prophecies of swift judgment it reveals God's abhorrence of persistent sin. It warns that idolatry, forgetting God, and continued apostasy bring divine anger, judgment, and suffering as the consequence of unfaithfulness.
It teaches that Israel was to trust in God alone rather than depend on other peoples or seek help and deliverance from those who did not serve Him.
The same warning about forsaking God applies to leaders of the Northern army turning from the God of Israel to deceptive spirits instead of trusting Him for deliverance.
The same pattern of idolatry, forgetting God, and provoking His anger applies to the world's rebellion, which is linked to disregard for God's law and Sabbath.
The warning was fulfilled when Israel departed from God and the Jewish nation was taken into captivity and scattered, the Temple was destroyed, and its sacrifices and ceremonies ceased.