Charles Ellicott Commentary Exodus 32:27

"And he said unto them, Thus saith Jehovah, the God of Israel, Put ye every man his sword upon his thigh, and go to and fro from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbor." — Exodus 32:27 (ASV)

Thus says the Lord God.— Moses felt that he was divinely commissioned to perform this act of severity. The lives of all who had committed the idolatry were justly forfeit. Trial was unnecessary where the offence was being openly committed before the eyes of all. Such dancing and such shouting could not possibly be Jehovah-worship. It was by its very character idolatrous.

Go in and out from gate to gate...— that is, “pass through the whole camp from end to end, visit all parts of it, and wherever you see the rites continuing, smite with the sword—smite, and spare not.”

Slay every man his brother.— Compare Exodus 32:26. The Levites who had rallied to the call of Moses might find their own brothers or their own sons among the idolaters. If they did, they were still to smite, though the offender was their near relative.