Deuteronomy 4:3

3.Your eyes have seen what the Lord did. This enlargement more clearly shews that so conspicuous was the example given in the punishment, that it could not be hidden from even the most ignorant; for Moses does not here address those of refined judgment, but the common people generally, who had only been spectators. Assuredly, if God’s vengeance had been less manifest, he would not have so confidently appealed to them as witnesses; hence was their stolidity the less excusable, if they were blind to so plain and notorious a fact.

His praise of their constancy I refer to the present case alone; for it is abundantly clear that they did not persevere in cleaving to God. The meaning is, that there was a manifest discrimination in this Divine chastisement, so that the death of the ungodly multitude should preserve the pure worship of God among the survivors.