That is, it is necessary, in accordance with God’s plan (see Ps. 110:1; Matt. 22:43, 44), for Christ to continue reigning until the complete subjugation of all God’s enemies. That it is Christ who subjugates the opposition is clear from 1 Cor. 15:24. Verses 27, 28 show that He does this under the Father’s mandate.

 That is, the Father (v. 28).

 This corresponds to the “footstool” of Ps. 110:1, from which psalm the apostle freely quotes.