Charles Ellicott Commentary Joel 3:21

"And I will cleanse their blood, that I have not cleansed: for Jehovah dwelleth in Zion." — Joel 3:21 (ASV)

I will cleanse...—The Septuagint translates this sentence, I will avenge their blood, and not leave it unavenged—i.e., the innocent blood mentioned in Joel 3:19; but the promise seems rather to indicate, as in the English Version, the extension of God’s pardon to those previously unpardoned.

The Lord dwells in Zion — i.e., over a raging and swelling world, probably unconscious of Him, the Lord nevertheless reigns in the heavenly Jerusalem, and all His redeemed shall dwell securely under His eternal rule. And the name of the city from that day shall be Jehovah Shammah, the LORD IS THERE (Ezekiel 48:35).