28 And the children of Levi did according to the word of Moses: and there fell of the people that day about three thousand men.

30 And it came to pass on the morrow, that Moses said unto the people, Ye have sinned a great sin: and now I will go up unto the Lord; peradventure I shall make an atonement for your sin.

It is not entirely clear who the three thousand killed by the Levites were. This verse and the following ones suggest that there still remained some of those Israelites who had participated in the act of idolatry. Indeed, if the entire camp, (or even just a significant fraction of the camp) had participated, then there would have been anywhere from tens of thousands to millions of idolaters still standing after the previous day’s slaughter.

It may be that when Moses returned the majority of the people were ambivalent as to whether or not they would return to serving the Lord, but that three thousand stood against Moses, and were prepared to defend their licentiousness by force. I think it is notable that the three thousand that were slain were “men,” which sounds to me like it might have been an army of rebels that had to be put down before the rest of the people gave way to Moses.

In either case, it seems clear from verse 30 that there were still many left alive who were soiled with sin. Moses, who had been the fierce sword of justice on the day before, now became the merciful advocate. Having seen for himself the weight of the people’s sin, he would return yet again to the mountain, in order to plead their case before the Lord.