7/5/13 V. 18, The Lord says to the wicked, like he says to each of the seven churches in Rev 2 and 3, that he knows their deeds and also their thoughts, which is a problem because their deeds and thoughts are evil and anti-God. Therefore, warns God, the time is coming when he will gather everyone to see his glory. This would have been fulfilled for Israel had they followed the divine plan, but will still be fulfilled in an end-time scenario. This eschatological gathering is a major theme in Revelation (Rev 14:18, 16:14-16, 19:17-19, 20:8).
V. 19, However, before the great gathering for the Day of God Almighty, there’s another gathering when people come out of Babylon as the gospel goes to the world. God promises to set (leave in LXX) a sign among the rebellious. That sign will be those who have escaped from Babylon. In other words, it seems like the scenario is that those whom God called out of Babylon will be left as an ongoing message to those still in Babylon who haven’t yet come out because they haven’t yet heard of the Lord or seen his glory.
The reason Babylon seems to figure here is because Tarshish, Put, Lud, Meshech, Tubal, and Javan and the islands or coastlands are also listed in Eze 27:10-13. In that context the message is to Tyre, which we have already seen in the comments on Isaiah 23:1 is synonymous with Babylon.
So God’s true and loyal people will declare God’s glory among the nations.
V. 20, And this gospel initiative will convince many to leave Babylon. They will come to worship God as he commands, borne in celebratory fashion to Jerusalem. This surely has connection with the Armageddon scenario in Revelation where the three angels’ messages in Revelation 14 go out to the world and the warning goes out in Rev 18:4 to “come out of her my people.”
In Rev 16:13-16, which is the primary Armageddon passage in Revelation, the three evil angels go out to gather the world for war on the great day of God Almighty. This is the counterfeit side of the three angels going out in Revelation 14. There is a spiritual war between the true people of God and the rebellious. The point of the war is to bring as many to one side or the other as possible. Many will be convinced by God’s “sign” of his people and will exit Babylon, but many will also remain in Babylon.