God will restore what the enemy has taken from his people, while those who've chosen the dark side watch. It says that they will know that the Lord is among his people. For those who say, "why does it say Israel?" remember, God has grafted us in, we are adopted, because Jesus chooses us.
You shall know that I am in the midst of Israel, and that I am the LORD your God and there is none else. And my people shall never again be put to shame. Joel 2:27
When comparing this portion of scripture to Revelation, God may have given Joel a vision pointing to the 1000 years of peace that coincides with God's restoration of everything the enemy has taken; the similarities are there. It says among the survivors will be those whom the LORD calls; that's us! Then comes the judgment. Thankfully we have Jesus who covers our sins; it will not be such a great day for those who rejected him.
And it shall come to pass that everyone who calls on the name of the LORD shall be saved. For in Mount Zion and in Jerusalem there shall be those who escape, as the LORD has said, and among the survivors shall be those whom the LORD calls. Joel 2:32