Why must Christ reign and how long does His reign last?

Christ must reign until He has put down all rule, authority, and power, and until all enemies are placed under His feet. Scripture says He then delivers the Kingdom to God the Father. His reign lasts until that final subjection is complete; then comes the end, when human history as we know it concludes.

Les explains that 1 Corinthians 15 places this reign after the resurrection order: Christ the firstfruits, then those who are Christ's at His coming—believers from Adam and Eve through the end of the Tribulation, including the Church's resurrection/rapture and the Old Testament and Tribulation saints. Then comes the end, when Christ delivers the Kingdom to God the Father. He must reign until He has put down all rule, authority, and power and has put all enemies under His feet. Les ties this to Psalm 110:1, a prophetic Psalm where God the Father speaks to God the Son: 'Sit thou at my right hand, until I make thy enemies thy footstool.' Les says the enemies made Christ's footstool are all the forces of opposition.

1 Corinthians 15:24

Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.

1 Corinthians 15:25

For he must reign, till he hath put all enemies under his feet.

Why must Christ reign until all enemies are put under His feet?

Christ's reign continues because all rule, authority, and power must be put down before He delivers the Kingdom to God the Father. Psalm 110 confirms the Father tells the Son to sit at His right hand until all opposition becomes His footstool, making every enemy subject to Him.