This is a good question! Before we seek to answer it, it is vital to see that God “cannot lie” (Titus 1:2) and He will never “contradict Himself.” Whatever these two passages are teaching us about His anger, it can’t mean He is saying one thing in one verse and then contradicting Himself in another.

In Jeremiah 17:1-4 God is condemning Judah for her IDOLATRY, as we see in verse 2, “Their altars and their wooden images, by the green trees on the high hills.” Because of their idolatry in those high hills He goes on to say in verse 3, “O My mountain in the field, I will give as plunder your wealth, all your treasures, and your HIGH PLACES OF SIN within all your borders.” He goes on in verse 4 to say, “And you, even yourself, shall let go of your heritage which I gave you; and I will cause you to serve your enemies in the land which you do not know; for you have kindled a fire in My anger which shall burn forever.” In short, God was pronouncing His judgment upon them; they would be conquered and taken to the foreign land of their enemies, along with all their wealth and treasures. This was literally fulfilled when the Babylonians invaded the land and took their property and survivors as captives to Babylon. They had caused “God’s anger to burn” and because they would not repent, His anger would not cease; in other words, His anger would result in judgment against those who had given themselves over to idols (false gods). I don’t believe the word “forever” here speaks of His “eternal anger,” (yet it is surely a foreshadowing of the time when God, in His righteous anger, will cast unsaved sinners into the lake of fire (Revelation 20:15 and 21:8).

In Deuteronomy 32:21-22 we see the same thing: “They have provoked Me to anger by what is not of God; they have moved Me to anger by their FOOLISH IDOLS. But I will provoke them to jealously by those who are not a nation; I will move them to anger by a foolish nation. For A FIRE IS KINDLED BY MY ANGER, and shall burn to the lowest hell; It shall consume the earth with her increase, and set on fire the foundations of the mountains.” Here too their “sin of IDOLATRY” had caused “God’s anger to burn.” His anger would not quit burning while they refused to repent of this terrible sin of worshipping “false gods.” But we must see that His judgment is not speaking of “eternal judgment” (for this judgment took place in TIME and ON EARTH), and thus His anger is not speaking of His anger that will one day cause Him to “cast unrepentant sinners into the lake of fire” (which will be forever). We must conclude that the word “forever” does not speak of the endless ages of eternity; rather, it speaks of His anger lasting “as long as they remained unrepentant.” They DID NOT REPENT, so the judgment came to pass!

Now let’s look at Micah 7:18-20, “Who is a God like You, pardoning iniquity and passing over the transgressions of the remnant of His heritage? He does NOT RETAIN HIS ANGER FOREVER, because He delights in mercy. He will again have compassion on us and will subdue our iniquities. You will cast all our sins into the depths of the sea. You will give truth to Jacob and mercy to Abraham, which You have sworn to our fathers from days of old.” We have seen God’s anger resulting in judgment upon Judah for their idolatry and unwillingness to repent, but this passage looks on to a future day when the whole nation WILL REPENT and God will PARDON THEIR INIQUITIES! At that time, they will have their eyes opened to see that Jesus Christ is their Messiah. They will see that when He was rejected by the nation (see John 1:11) and crucified, that He was NOT dying for His sins, but for their sins. They will cry out from the depths of their repentant hearts, “Surely He has borne our griefs and carried our sorrows; yet we did esteem Him stricken, smitten by God, and afflicted, BUT HE WAS WOUNDED FOR OUR TRANSGRESSIONS, HE WAS BRUISED FOR OUR INIQUITIES, and the chastisement for our peace was upon Him, and by His stripes we are healed. All we like sheep have gone astray; we have turned, every one, to his own way; and the Lord has laid on Him the iniquity of us all” (Isaiah 53:4-6). God’s anger will then be gone as will their sins! And His mercy will cause them to praise Him. So here too the word “forever” speaks of TIME HERE ON EARTH and not of eternity.  (DO)  (707.5)