The removal of Satan is the first necessary step to set up the great test for men (in the millennial kingdom): to see if they can live a blameless life— without being tempted by Satan. The idea is that with Satan and his demons out of the picture, unregenerate man will have a fair chance to live without sin (even though they will be born in sin).
And also, at the beginning, all unregenerate (unsaved) people will be killed, either in the battle of Armageddon (Rev. 19:15-21) or in the judgment known as the sheep and goat’s judgment (Matt. 25:31-46). Therefore, from the beginning of the millennial kingdom, there will be no unsaved people. All who go into the millennium will be Christian mortals who survive the tribulation. And there will also be many immortal, glorified believers, both from the Old Testament era and the New Testament era; but these (which includes me) will be like the angels; and our residence will not be on the earth, but in the New Jerusalem (Rev. 21:9-27).
So the beginning of the millennial kingdom will be quite glorious—with no Satan, no demons, and no unbelievers. However, since the mortal believers will still have their unredeemed flesh, they will be capable of sinning. This means, of course, that their offspring will be born in sin and they will need to choose to believe.
And this is what the test is about: whether they will choose to believe of not. And we already know that many will choose not to believe.
I wonder how it will be at that time. Can you imagine it? Without Satan to deceive the world and without the demons lurking around everywhere, what will it be like? How different will it be? The answer lies in how we view the problem of sin and temptation. Man’s choice of whether to sin or not is always with himself. He makes that choice of his own will. But though man in the millennium will still have his old sinful flesh hanging on to him, the choice, it would seem, would be easier and clearer, because the one who deceives and destroys will be removed. Moreover, I think any choice made for evil in the millennium will be more devastating, that is, the consequences will be more remorseful; for the devil, having been removed, will leave man with no excuse. Thus the blame for a sin will be made plain, pointing directly to the heart of man. Ah yes, all things will be made black and white. It will be clear to all what is good and what is evil, what is sinful and what is holy. For the removal of Satan will also remove all source of deception and trickery. If any evil is concocted, it will be fully known to all.