Q. What is the purpose of the Millennium? To prove man cannot/will not live without rebelling against God? I thought that everybody, God included, already knows that.
A. I think you’ve got it. Natural man cannot/will not live without rebelling against God. But I’m not convinced that everyone already knows that, so God will provide one more chance for man to prove himself worthy.
Remember this as well. In the millennium 3 things will be different than ever before. 1) It begins with only believers who have actually lived through the worst of God’s wrath. 2) Satan will be bound for the entire time. And 3) Jesus will reign over them in person.
In spite of all that, natural man will eventually rebel against the Lord’s rule. His sin nature makes it impossible for him to live in a manner pleasing to God, even in an ideal environment like the millennium. That’s why we have to be divested of our sin nature in the instant we’re taken in the rapture. For the perishable must clothe itself with the imperishable, and the mortal with immortality (1 Cor. 15:53).