Adam Clarke on Deuteronomy 7:12

The Lord - shall keep unto thee the covenant - So we find their continuance in the state of favor was to depend on their faithfulness to the grace of God. If they should rebel, though God had chosen them through his love, yet he would cast them off in his justice. The elect, we see, may become unfaithful, and so become reprobates. So it happened to 24,000 of them, whose carcasses fell in the wilderness because they had sinned; yet these were of the elect that came out of Egypt. Let him that standeth take heed lest he fall.