Revelation 20:1-4

1 THEN I saw an angel descending from heaven; he was holding the key of the Abyss (the bottomless pit) and a great chain was in his hand. 

2 And he gripped and overpowered the dragon, that old serpent [of primeval times], who is the devil and Satan, and [securely] bound him for a thousand years. 

3 Then he hurled him into the Abyss (the bottomless pit) and closed it and sealed it above him, so that he should no longer lead astray and deceive and seduce the nations until the thousand years were at an end. After that he must be liberated for a short time. 

4 Then I saw thrones, and sitting on them were those to whom authority to act as judges and to pass sentence was entrusted. Also I saw the souls of those who had been slain with axes [beheaded] for their witnessing to Jesus and [for preaching and testifying] for the Word of God, and who had refused to pay homage to the beast or his statue and had not accepted his mark or permitted it to be stamped on their foreheads or on their hands. And they lived again and ruled with Christ (the Messiah) a thousand years.

When this all happens the dead in Christ will have been raised up and transformed and those who were still alive in Jesus will have been transformed to be like Christ. We will now rule and reign with our Lord, Jesus! 

Beloved, we are [even here and] now God's children; it is not yet disclosed (made clear) what we shall be [hereafter], but we know that when He comes and is manifested, we shall [ as God's children] resemble and be like Him, for we shall see Him just as He [really] is. 1 John 3:2

When we are transformed to be just like Christ we will have the authority of Christ. We will act as judges and we will rule and reign with Jesus. This is why it is called the Millennial Reign of Christ, not just the Millennial reign of Jesus, because we are all part of Christ and we all rule together. We will rule because we are members of Jesus body, flesh of His flesh, bone of His bone, Ephesians 5:30. We will be doing what our Head is doing. There has to be people left after the Great Tribulation who weren’t killed and who never were chosen to be saved. These are the people who we rule and reign over. We rule and reign because we are one with Jesus, Who rules with an iron scepter during these 1,000 years. If there were only raptured people, only the Bride, alive during the Millennial Reign, would Jesus rule us, His Bride, with a rod of iron? 

We were shown that satan will be released and we will see that when he is, that he will lead astray a lot of people. So there has to be people left alive who are not the bride of Christ. If there isn’t, then our Father would have had to create some more unrighteous people for us to rule over, or we would have had to birth them.