I have a question that I just can’t seem to find the answer to, so I’ll just jump right into it- I don’t understand the Millennium period- do we as saved Christians come back in our heavenly bodies to dwell with the tribulation saints? And I also read that God removes all non believers from the earth when he comes back but yet at the end of the Millennium there is as many as the grains of sands ready to fight with Satan? And when a tribulation Saint dies a natural death during that time do they instantly get their heavenly bodies?
Church Age believers will dwell in the New Jerusalem during the Millennium. Surviving Tribulation believers will live on Earth. God does remove all living unbelievers from Earth in the sheep and goat judgment at the 2nd coming (Matt. 25:31-46), so it’s the offspring of Tribulation believers who join Satan in the final rebellion. The Bible doesn’t mention the final destiny of Tribulation believers who enter the Millennium. And while I can’t imagine there won’t be any, it doesn’t say anything about people being saved during the Millennium either.
The New Testament contains many teachings that are specific to the Church. Some of them were hinted at in the Old Testament and others were new revelation given to Paul and others in the early days of the Church. Based on that precedent, as well as the Bible’s scarcity of information about life in the Millennium, I’m guessing the Lord will have additional revelation for man in the early days of the Millennium like He did for us. If so, questions like the ones you’ve raised will be answered at that time.