John Calvin on Joel 3:20

God here testifies that his redemption would not be for a short time, but that its fruit would be for a long, period, yea, perpetual: for it would be but a small thing for the Church to be redeemed, except God kept it safe under his own power. This second thing the Prophet now adds, — that Judah shall always remain safe, and that Jerusalem shall be for a continued succession of ages. The ungodly, we know, sometimes flourish for a time, though before God they are already doomed to destruction. But the Prophet here declares, that the fruit of the redemption he promises will be eternal: for God is not led to deliver his Church only for a moment, but he will follow it with perpetual favor, and remain constant in his purpose and ever like himself; he is therefore the eternal and faithful protector of his people. The last verse follows —

Joel 3:21

21. For I will cleanse their blood that I have not cleansed: for the Lord dwelleth in Zion.

21. Et mundabo sanguinem corum, non mundavi: et Jehova habitans in Sion.