‘Now I desire to put you in remembrance, though you know all things once for all, that the Lord, having saved a people out of the land of Egypt, afterwards destroyed those who believed not.'
His first example is of an Israel who had been ‘saved' out of the land of Egypt. He wants them to remember what happened to those people who had supposedly been saved out of Egypt. Apart from a few they had refused to believe.
The only incident in Exodus to Deuteronomy which speaks of the people ‘not believing' is when they were faced up to entry into Canaan and refused to go forward. See Numbers 14:11; Deuteronomy 1:32; Deuteronomy 9:23. They had left Egypt ‘believing' (Exodus 14:31), but on their refusing to advance on Canaan God declared, ‘how long will they not believe in Me?' Furthermore in Deuteronomy 9:23 Moses declares, ‘when the LORD sent you from Kadesh Barnea, saying, “Go up and possess the land which I have given you”, then you rebelled against the commandment of the LORD your God, and you believed Him not nor listened to His voice'. And the same is true in Deuteronomy 1:32 in context. So this is the great incident of specific and stated unbelief in the life of Israel.
And what was the result? That God swore that apart from Moses, Caleb and Joshua (who had believed) every adult from twenty years old and upwards (Numbers 14:29) who were involved in the unbelief would die in the wilderness. And that was what happened. They died to the last man (Deuteronomy 2:16). ‘He destroyed those who believed not'.
The lesson is clear, that, whatever the claims about having ‘been saved', without a true belief that responds to God Himself, a God active to save, the God of the Old Testament as well as the New, all who do not believe will perish.