John Calvin Commentary Joel 3:20

"But Judah shall abide for ever, and Jerusalem from generation to generation." — Joel 3:20 (ASV)

God here testifies that His redemption would not be for a short time, but that its fruit would be for a long period, indeed, perpetual. For it would be only a small thing for the Church to be redeemed, unless God kept it safe under His own power. The Prophet now adds this second point — that Judah shall always remain safe, and that Jerusalem shall be for all generations.

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 does not deliver His Church only for a moment, but He will follow it with perpetual favor, remain constant in His purpose, and always be true to Himself; He is therefore the eternal and faithful protector of His people.