Deuteronomy 8:19

And it shall be, if thou do at all forget the Lord thy God,
&c.] Either the mercies they received from him, not acknowledging they came from him, but ascribing them to themselves; or their duty to him, to whom they were so greatly obliged: and walk after other gods, and serve them, and worship them; which would be to forget him indeed, forsaking his worship, and giving homage and adoration to idols, which is what is intended by these expressions:

I testify against you this day that ye shall surely perish;
by one judgment and calamity or another, as the sword, famine, pestilence, and captivity; there being nothing more provoking to God than idolatry, which so much detracts from his honour and glory: and which besides, in such a people, so highly favoured of God, it argued the basest ingratitude.