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.
Les explains that in verse 24, at the end—when time as we know it and human history close—Christ will have delivered up the Kingdom to God, even the Father, and will have put down all rule, authority, and power. The reason given is that He must reign until He has put all enemies under His feet. Les then turns to Psalm 110:1, which he treats as prophetic, where the Lord says to my Lord, "Sit thou at my right hand, until I make thy enemies thy footstool." He identifies this as God the Father speaking to God the Son, matching other references to Christ sitting at the Father's right hand. The enemies made Christ's footstool are all the forces of opposition.
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.
For he must reign, till he hath put all enemies under his feet.