John Calvin Commentary Deuteronomy 4:3

"Your eyes have seen what Jehovah did because of Baal-peor; for all the men that followed Baal-peor, Jehovah thy God hath destroyed them from the midst of thee." — Deuteronomy 4:3 (ASV)

Your eyes have seen what the Lord did. This further explanation more clearly shows that so obvious was the example given in the punishment, that it could not be hidden even from the most ignorant. For Moses does not here address those of refined judgment, but the common people generally, who had only been spectators. Certainly, if God’s vengeance had been less evident, he would not have so confidently appealed to them as witnesses. Therefore, their dullness was all the less excusable if they were blind to so plain and obvious a fact.

I understand his praise of their constancy to refer to this present situation alone, for it is abundantly clear that they did not persevere in clinging to God. The meaning is that there was an evident distinction in this divine chastisement, so that the death of the ungodly multitude would preserve the pure worship of God among the survivors.