God’s purposes will not be thwarted (v. 12-21). And they are glorious (1-4): there will be no war (4), because disputes between nations will be decided by the law of God (3), from His city, Zion.
The political sphere will changed by a change first in worship: the nations will worship God in Zion, and God will rule them through His law and His new and better David. And of the increase of peace and his government, there will be no end (9:7).
Zion is His people. We will reign with him, as Adam and Eve were meant to do. This is the future of the people of God. Yet Judah (we?) live on an opposite trajectory (6), just like your average Philistine. They are full of idols, which they will throw away as they run for the hills under God’s wrath (20). And Judah has become like them - kings, living like paupers.
So the command is threefold. First, we must repent from being typical, just like the nations. Second, we must “walk in the light of the Lord” (5), submitting all of our lives to His Word, which is the light to our path (Psalm 119:105). And thirdly, we must stop “regarding” or fearing man (22). So much idolatry arises from the fear of man, instead of fearing God.
Believe in God’s promised future for you, and that of the nations, and walk now, as you will be, one day soon.