Kingly office of Christ - (Easton's Bible Dictionary)
One of the three special relations in which Christ stands to his people. Christ's office as mediator comprehends three different functions, viz., those of a prophet, priest, and king. These are not three distinct offices, but three functions of the one office of mediator.
Christ is King and sovereign Head over his Church and over all things to his Church (Eph. 1:22; 4:15; Col. 1:18; 2:19). He executes this mediatorial kingship in his Church, and over his Church, and over all things in behalf of his Church. This royalty differs from that which essentially belongs to him as God, for it is given to him by the Father as the reward of his obedience and sufferings (Phil. 2:6-11), and has as its especial object the upbuilding and the glory of his redeemed Church. It attaches, moreover, not to his divine nature as such, but to his person as God-man.
Christ's mediatorial kingdom may be regarded as comprehending, (1) his kingdom of power, or his providential government of the universe; (2) his kingdom of grace, which is wholly spiritual in its subjects and administration; and (3) his kingdom of glory, which is the consummation of all his providential and gracious administration.
Christ sustained and exercised the function of mediatorial King as well as of Prophet and Priest, from the time of the fall of man, when he entered on his mediatorial work; yet it may be said that he was publicly and formally enthroned when he ascended up on high and sat down at the Father's right hand (Ps. 2:6; Jer. 23:5; Isa. 9:6), after his work of humiliation and suffering on earth was "finished."