Albert Barnes Commentary Joel 2:18

"Then was Jehovah jealous for his land, and had pity on his people." — Joel 2:18 (ASV)

Then will the Lord be jealous for His land - Upon repentance, all is changed. Before, God seemed set upon their destruction. It was His great army which was ready to destroy them; He was at its head, giving the word. Now He is full of tender love for them, which resents injury done to them, as done to Himself.

The word could perhaps be more strictly rendered, “And the Lord is jealous.” He would show how instantaneous the mercy and love of God for His people is, restrained while they are impenitent, flowing forth upon the first tokens of repentance.

The phrase “jealous for,” when used of God—as in jealous for My holy Name (Ezekiel 39:25) and jealous for Jerusalem (Zechariah 1:14; Zechariah 8:2)—is used, when God resents evil which had been actually inflicted.