Albert Barnes on 1 Corinthians 15:25

For he must reign. It is fit, or proper, (δει,) that he should reign till this is accomplished. It is proper that the mediator kingdom should continue till this great work is effected. The word "must" here refers to the propriety of this continuance of his reign, and to the fact that this was contemplated and predicted as the work which he would accomplish. He came to subdue all his enemies. See Ps 2:6-10, 90:1, "The Lord said unto my Lord, Sit thou at my right hand until I make thine enemies thy footstool." Paul, doubtless, had this passage in his eye as affirming the necessity that he should reign until all his foes should be subdued. That this refers to the Messiah is abundantly clear from Mt 22:44,45.

(a) "he must reign" Ps 2:6-10, 45:3-6, 90:1, Eph 1:22, Heb 1:13