You mention that there will be those who fight in a war against Christ at the end of the Kingdom. How and why would believers in the Kingdom oppose Christ?
On the opening day of the Millennial Kingdom, the world will consist of believers only, but these believers will exist in two ways: resurrected human beings in eternal, sinless bodies and natural-born human beings in temporal, sinful bodies
The resurrected human beings will be those who died or were Raptured prior to the start of the Kingdom. They receive new resurrected, sinless bodies prior to the start of the Kingdom, both Old Testament and New Testament saints, and enter the Kingdom in that form. These human beings will never sin again, but they are not the only human beings to enter.
The world will also be populated initially by natural-born human beings who lived through the Tribulation, and therefore they enter the Kingdom in sinful, earthly bodies like those we occupy now. These sinful individuals are believers, so they may enter the Kingdom, but they continue to live in the same way we do now: they will marry, have children, and their offspring will also have sinful earthly bodies. The resurrected human population will reign over the natural-born population, which must be ruled because of their sinful nature.
These believers who enter the Kingdom coming out of Tribulation are the “sheep” separated from the “goats” in Matthew 24. Since these believers never died prior to the start of the Kingdom, they were never resurrected into glorified bodies. Therefore, they enter into the Kingdom in the nature of Adam and are able to reproduce as we do today. Their descendants give rise to an unbelieving population in the Kingdom. (You can learn about these two populations in detail by listening to our Revelation Bible study on our website.)
In Revelation 20 we’re told that at the end of the age, the enemy will be released so he can deceive the nations of natural descendants living in the Kingdom to rebel against Christ’s rule:
Rev. 20:7 When the thousand years are completed, Satan will be released from his prison,
Rev. 20:8 and will come out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together for the war; the number of them is like the sand of the seashore.
Rev. 20:9 And they came up on the broad plain of the earth and surrounded the camp of the saints and the beloved city, and fire came down from heaven and devoured them.
Rev. 20:10 And the devil who deceived them was thrown into the lake of fire and brimstone, where the beast and the false prophet are also; and they will be tormented day and night forever and ever.
Notice that the natural (i.e., unsaved) population of the Kingdom will grow over the thousand years to reach the point of an uncountable number. They will come against Christ, yet He will prevail and bring Satan and his forces. For a complete understanding of the Kingdom, please listen to Lessons 20A-20D of our Revelation study.