Christ, the mediator of the Covenant of Grace, executes the offices of prophet, priest, and king of His church. Christ, the prophet, reveals to His church God’s will for its salvation, edification, and administration. As priest, He reconciles and intercedes for His people. His kingship involves establishing, ruling, and preserving that people for Himself. The offices are coextensive. Being the redeemer of God’s elect, Christ is king of those of whom He is prophet and priest.
Kingship involves two basic functions. One is to establish an environment in which a people may prosper. The other is to see to their defence. The environment of Christ’s mediatorial kingship is the visible church. He has established elders acting in graded elderships to govern it and has given to that government laws and the power of the keys for its administration. He also rules over His individual subjects directly, bestowing upon them saving grace and disciplining them. As king, Christ engages in a just war for His people’s preservation and defence.
Historically, Christ’s kingship of the church has been undermined from two directions. The first is the Papacy which has taken to itself the prerogatives of Christ and placed them into the hands of an earthly individual who acts in Christ’s name. The second is Erastianism which subsumes the government of the church under that of the state and arrogates to itself the power of the keys.
The relationship of Christ’s mediatorial kingship to those outside of the church has been expressed in more than one way. Some have spoken of a mediatorial kingship of Christ over the nations. However, in order not to subvert the coextensiveness of Christ’s offices, most of them have changed the definition of mediatorial from that which specifically concerns the work of Christ, the Mediator of new covenant and Redeemer of God’s elect, to that which, more broadly, concerns the person of Christ, the Son of God incarnate. Others, by far the majority, have drawn a distinction between that which Christ rules over as the Second Person of the Trinity and that which He rules over as mediator.
While much of this discussion now rests upon semantics, it has in the past had practical implications. Those holding to the mediatorial kingship of Christ over the nations have in some instances sought to subsume the government of the church under that of the state in a Christian country. In other instances, they have held governments and constitutions to be illegitimate and unlawful because they do not officially recognise the mediatorial rule of Christ over the nation, demanding that believers dissent from political activity in those countries.
There is one point which comes out of the mediatorial kingship discussion which catches the attention. By what authority does Christ wage war on His enemies and those of His people? Does He require a mediatorial dominion over them in order to subdue them? All that is needed to subdue His and His people’s enemies is power. The only legitimate authority to exercise that power which He requires is His kingly obligation to defend His own people from attack.
On the whole, it is simpler, in line with the Westminster Standards, to restrict the use of the language of mediatorial kingship to Christ’s relationship to His people in terms of the covenant of grace.