Deuteronomy 32:15
But Jeshurun waxed fat, and kicked: thou art waxen fat, thou art grown thick, thou art covered with fatness; then he forsook God which made him, and lightly esteemed the Rock of his salvation. But Jeshurun waxed fat, and kicked: thou art waxen fat, thou art grown thick, thou art covered with fatness; then he forsook God which made him, and lightly esteemed the Rock of his salvation.
The verse stands in the song of Moses, which rehearses how God has dealt with His people in the past and foreshadows future events, including the final victory of the faithful when Christ comes again in power and glory. God commanded Moses to frame that song and teach it to Israel and to their children so the past would not be forgotten, preserving the history of His care and deliverance as a lasting reminder of His dealings with them.
That remembrance was to be active and visible, for God frequently set up waymarks in His dealings, the song was to be taught by parents to children, and memorials were to be gathered up and kept in sight and specially preserved so that when children inquired the whole story of His providential dealings and marked goodness and mercy in care and deliverance could be repeated. The history is to be often brought before young and old, with His goodness recounted and praised for His wonderful works. As the Fountain of life and power He has done for His chosen people what should inspire every heart with thanksgiving, He can make the wilderness a fruitful field for those who keep His commandments for the glory of His name, He desires His obedient children to claim His blessing with praise and thanksgiving showing they have reason for joy and gladness, and He is grieved that so little praise is offered.
To wax fat and kick means to grow prosperous and self-satisfied and then turn against God. That turning meant forsaking God who made Israel and treating with contempt the Rock of salvation, being unmindful of the Rock that begot and forgetting the God that formed him. The Rock of salvation is Christ.
Forsaking God in this way led to idolatry, provoking Him to jealousy and anger with strange gods and abominations by sacrificing to devils and not to God, to gods they did not know, to new gods that newly came up whom their fathers did not fear. The surrounding song reveals both God's love for His people and His hatred of sin, and warns that persistent rebellion brings swift calamity and judgment. Those warnings of coming judgment made Josiah tremble for the future of Judah as its apostasy continued. The verse illustrates how disregard for God's law, including the Sabbath, leads to rebellion against God.