But what exactly does this mean? When we talk about Christ’s kingdom, we have to distinguish before we discuss. In Francis Turretin’s (d. 1687) words, “…Before all things we must distinguish the twofold kingdom….”
Below is my edited summary of Turretin’s helpful distinction of Christ’s twofold kingdom. (I’m not sure how to put it into a nice chart; perhaps some formatting expert can do that on his/her own.)
One kingdom of Christ is natural or essential. He is king over all creatures with glory and majesty equal to that of the Father and the Holy Spirit. This kingdom extends over all creatures and is founded on the decree of providence. This kingship is exercised by Christ inasmuch as he is God and the Logos. This kingship belongs to Christ by nature, which is why this kingdom is his natural kingdom. (Note: Many Reformed theologians also call this the kingdom of power.)
The other kingdom of Christ is mediatorial and economical (that is, having to do with the economy of salvation). He exercises this kingship in a peculiar manner as God-man; it has everything to do with the church. It is founded upon the decree of election. It is called his mediatorial kingship because it is a dominion peculiar to the Mediator according to grace and salvation. God constituted Christ as King over the church. (Note: Many Reformed theologians also call this the kingdom of grace/glory.)
This distinction of Christ’s twofold kingdom is helpful to remember especially when reading the Reformed confessions where they discuss the second petition of the Lord’s Prayer (your kingdom come…). From Thomas Watson to Thomas Vincent to Zacharias Ursinus to Wilhelmus’ a Brakel, you’ll notice Reformed theologians saying that “your kingdom come” is a prayer which specifically has to do with Christ’s mediatorial kingship – his gracious and salvific reign over the church.