Christ as the Son of Man will receive authority from the Father to execute judgment in His humanity according to the will of the Father who sent Him (John 5:27-30). God has given all judgment to Christ because He is the Son of Man. The apostle Paul said that God has appointed this one man to judge the living and the dead (Acts 10:42; 17:31; 2 Tim. 4:1). He will judge all the dead people to decide who should be resurrected at His coming and who should be resurrected one thousand years later. At His coming all His believers who are dead will be resurrected unto life before the millennium (John 5:29a), and all the unbelievers who are dead will be resurrected unto judgment after the millennium (v. 29b).
He will judge all the people, signified by the sheep and the goats, who are living after His coming back (Matt. 25:31-46). During the great tribulation, Antichrist will persecute the believers. At that time, an eternal gospel will be preached to the nations (Rev. 14:6-7 and notes). Those who obey that gospel and treat the Jews and the suffering believers well will be blessed, will be reckoned as righteous, and will inherit the kingdom (Matt. 25:34, 37). These are the sheep. But those who do not will be cursed (v. 41) and will perish for eternity. These are the goats. The Lord will sit on the throne of glory at His return to separate the sheep from the goats. The sheep who have well-treated the Jews and the believers in the great tribulation will go into eternal life to be the nations of the judged world (vv. 33-40, 46b). They will be saved, not to receive eternal life but to enter into eternal life. Eternal life will be the sphere and the realm into which they will enter and in which they will live. They will be the nations around the New Jerusalem but not the kings. We believers, who are the New Jerusalem, will be the royal family to rule over all the nations. The goats who have ill-treated the Jews and the believers in the great tribulation will go into eternal punishment (the lake of fire) to perish (vv. 41-46a). Later, the Lord as the Son of Man will judge all the dead people who will be resurrected unto judgment after the millennium (Rev. 20:11-15).
Christ will reign in His humanity in the millennium, sitting on the throne of His glory with His faithful followers (Matt. 19:28-29).