Why is it that the doctrine of Christ’s Two Kingdoms, and his Mediatorial Kingdom being only the Church, became so prevalent in early Reformed theology, per Dr. Richard Muller:
‘The Reformed… tend to attribute the regnum universale [universal Kingdom] specifically to the Second Person of the Trinity and only the regnum oeconomicum [economic reign, including the Mediatorial Kingdom] to the God-man as Mediator.’
– Dictionary of Latin and Greek Theological Terms, pp. 259-260
The Westminster divine George Gillespie explains, in part, the historical answer to this below on this page. Here is a summary:
In the early Church, Photius (†376) denied the divinity of Christ. He did this by interpreting every passage that speaks of Christ as divine Lord receiving divine worship, as referring to Christ only as Mediator, and that as vice-regent, governing in the place of, God. Thus, Christ who, according to Photius, was not God, was worshipped as God because, as Messiah, he ruled in the place of God in the one, universal Mediatorial Kingdom (Christ not having a universal, divine Kingdom). His followers were known as Photinians.
After the Reformation, the Socinians (from Socinus, †1604) also denied the divinity of Christ. They followed the Photinians in asserting that Christ did not have a divine Kingdom as God, but only one, universal Mediatorial Kingdom, in which Kingdom he was subordinate to the Father (‘My Father is greater than I,’ Jn. 14:28).
This gave the occasion after the Reformation for orthodox Protestant writers (not just Reformed, but virtually all Protestants, Lutherans included), to assert, distinguish and emphasize Christ’s Two Kingdoms that they found in Scripture:
(1) One Kingdom as divine God in common with, and co-equal to, the Father and Holy Spirit, by creation, over all creation; and
(2) One Kingdom as the God-man, Mediator, only over his Church that the Father sent Him to save (Christ serving a role necessarily subordinate to the Father in order to accomplish redemption in taking up human flesh (Isa. 52:13, etc.).
The many early orthodox Protestant writers, as well as Gillespie, in order to distinguish and defend Christ’s essential Deity from his subordinate, economic role as Mediator, gave numerous reasons why these Two Kingdoms must be distinct. Not the least persuasive is because:
Christ’s divine Kingdom over all things can logically exist apart from there ever being a Mediatorial Kingdom of redemption. Christ cannot but have a divine Kingdom given God’s choice and act of creation. However, God could have created and yet not chosen to redeem any of his creatures, and thus there would be no Mediatorial Kingdom, as there was no necessity for God to save, it stemming only from his free and sovereign good pleasure. If one of Christ’s Kingdoms can exist without the other, then Christ has Two Kingdoms.
Some Erastians (from Erastus, †1583, who asserted the civil government’s authority over the Church), taking over the ground left by the Socinians, though acknowledging the divinity of Christ, likewise held that Christ only had one Kingdom: his Mediatorial Kingdom which they considered to be universal over all things. Other Erastians, a bit more careful, while admitting that Christ has two Kingdoms, yet made Christ’s Mediatorial Kingdom to be over all things, Christ as redemptive Mediator and God-Man, taking over, by delegation, God’s operations purely as the natural Creator. On either Erastian view, as Gillespie argues, this is a confusing and mixing of Christ’s necessarily distinct Two Kingdoms.
As Christ’s Two Kingdoms have different sources of origin and power from Christ, their administration in time and human society, and representation by headship of subjects, must necessarily be different. Gillespie, in Book 2, ch. 5 of Aaron’s Rod distinguishes nine ways in which the Two Kingdoms are different, including the subjects of Christ’s divine Kingdom necessarily including all created persons, while the subjects of Christ’ Mediatorial Kingdom necessarily including only the Church.
This history and its attendant theological implications is all in part why the doctrine of Christ’s Two Kingdoms was so widely prevalent shortly after the Reformation. It ought not to be inferred, though, that this Biblical doctrine was simply a reaction against Socinianism: The third generation reformers recognized and derived these concepts from careful exegesis of the Scriptures in contrast to subtle, but powerful, errors. The historic Reformed theologians had ample opportunity to sift, clarify, and fully treat the scriptural doctrine of the nature and extent of Christ’s Mediatorial Kingdom in response to the competing erroneous paradigms, not only of the Socinians, but also of the Roman Catholics, the Anabaptists, the Lutherans and the Erastians. By the time Gillespie came around, he was simply defending the established, fully worked out, Biblical and Reformed viewpoint.
Please enjoy Gillespie.
George Gillespie Aaron’s Rod Blossoming, book 2, ch. 5, p. 90-91 1646
The controversy draws deeper than he [Hussey, an Erastian] is aware of, for Socinians and Photinians, finding themselves puzzled with those arguments which (to prove the eternal Godhead of Jesus Christ) were drawn from such scriptures as call Him, ‘God, Lord, the Son of God;’ also from such scriptures as ascribe worship and adoration to Him, and from the texts which ascribe to Him a supreme lordship, dominion, and kingdom over all things,
(for this has been used as one argument for the Godhead of Jesus Christ and his consubstantiality with the Father, ‘The Father reigns, the Son reigns, the Holy Ghost reigns;’ see the book, Isaac Clare, Against Various Arians)
thereupon they [the Photinians and Socianians] devised this answer: That Jesus Christ, in respect of his kingly office, and as Mediator, is called God, and Lord, and the Son of God (of which see Fest. Honnii Specimen Controv. Belgic., p. 24; Jonas Schlichtingius, Contra Meisnerum , p. 436 [Schlichtingius was a Socinian]); and that in the same respect he is worshipped, that in the same respect he is King, and that the kingdom which the Scripture ascribes to Jesus Christ, is only as Mediator and Head of the Church, and that he has no such universal dominion over all things as can prove him to be the eternal Son of God.
This gave to orthodox Protestant writers more fully and distinctly to assert the great difference between that which the Scripture says of Christ as He is the eternal Son of God, and that which it says of Him as He is Mediator; and particularly to assert a two-fold Kingdom of Jesus Christ, and to prove from Scripture that, besides that Kingdom which Christ has as Mediator, He has another Kingdom over all things, which belongs to Him only as He is the eternal Son of God.
This the Socinians to this day do contradict, and stiffly hold that Christ has but one Kingdom, which he exercises as Mediator over the Church, and in some respect over all things; but by no means do they admit that Christ, as God, reigns over all things.
But our [Protestant and Reformed] writers still hold up against them the distinction of that two-fold Kingdom of Jesus Christ, see The Photinianism of Josua Stegmann, whose Photinian Errors are Succinctly Refuted in 56 Brief and Comprehensive Disputations, disputation 27, question 6.
[Stegmann (1588-1632) was a Socinian. For many futher historic, Protestant refutations of Photinianism, see Howard Malcom, Theological Index, p. 358.]
The same distinction of the two-fold Kingdom of Christ, as God and as Mediator, is frequently to be found in Protestant writers, see [The Leiden Professors,] A Synopsis of Pure Theology, Disputation 26, Thesis 53; Franciscus Gomarus (d. 1641), in Obadiah, the last verses; the late English Annotations on 1 Cor. 15:24, and many others. Let Amandus Polanus speak for the rest (A System of Christian Theology , Book 6, Ch. 29 [the Latin is given by Gillespie in a footnote]). See also the same distinction cleared and asserted by Mr. Willelm Apollonius, in his Right of the Magistrate Around the Sacred , part 1, p. 33 ff.