John Chrysostom
"Nor let us commit fornication, as some of them committed fornication." Why does he again mention fornication here, after speaking so much about it before?
Paul commonly acted in this way. When rebuking many sins, he enumerated them in order and indicated each one separately. Then, when speaking about others, he also recalled what had already been mentioned.
God acted in exactly the same way in the Old Testament. At every transgression He reminded the Jews of the calf and set that sin before their eyes.
Paul acts similarly here. He reminds them of this sin and also teaches that this evil likewise comes from sensual pleasure and gluttony.
Therefore he adds: "Nor let us commit fornication, as some of them committed fornication, and in one day twenty-three thousand fell."
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Why does he not say what punishment befell the Jews for idolatry? Either because it was known and evident, or because the punishment was not as great as in the time of Balaam.
Then they served Baal-Peor, having been drawn into impiety by the Midianite women who entered their camp on Balaam's counsel.
That this was Balaam's treacherous counsel is clear from the words of Moses, who says at the end of Numbers: "Along with their slain, they killed Balaam the son of Beor with the sword" in the Midianite war.
And they "took... plunder... And Moses was angry... and said, 'Why have you kept all the women alive? Behold, these, by Balaam's counsel, caused the children of Israel to turn away from the Lord in the matter of Peor'" (Num 31:8-9, 14-16).