Charles Spurgeon Commentary Exodus 32:28-29

"And the sons of Levi did according to the word of Moses: and there fell of the people that day about three thousand men. And Moses said, Consecrate yourselves to-day to Jehovah, yea, every man against his son, and against his brother; that he may bestow upon you a blessing this day." — Exodus 32:28-29 (ASV)

And there fell of the people that day about three thousand men. For Moses had said, Consecrate yourselves to day to the LORD, even every man upon his son, and upon his brother; that he may bestow upon you a blessing this day.

Such a colossal crime as that must be expiated before the Lord could again bless the chosen race.