1,) The most obvious version of two Kingdom theology which is perfectly necessary in order to be orthodox is the view that speaks of the Kingdom of Satan vs. the Kingdom of God.
13 He has rescued us from the dominion of darkness and brought us into the kingdom of His beloved Son, 14in whom we have redemption, the forgiveness of sins
There are two dominions / Kingdoms. One either belongs to the dominion of darkness or one belongs to the Kingdom of Christ. Here we find Augustine’s great work “The City of God,” wherein he describes these two Kingdoms as the City of God and the City of man. Of course, these two co-exist together at the same time on planet earth and because of the antithesis between the two, there is a conflict between these two kingdoms.
Now, keep in mind here that the kingdom of God, in this understanding, is not limited to the Church in terms of its presence. The kingdom of God is present wherever men are living in subjection to Christ regardless of whether that is in Church, Business, Family life, Education, Law, etc.
If one insists that the kingdom of Christ is only limited to the Church realm that is a different two kingdom than Augustines.
2.) A subsequent version of two kingdoms that can live alongside the #1 is the two kingdom theology which bespeaks the idea of internal vs. external. The former is invisible/spiritual while the latter is visible/material. The former applies to God’s immediate authority over the consciences of believers — what is sometimes called the rule of Christ in the hearts of His people. The external kingdom appeals to the mediate authority of Christ as incarnated through delegated human authorities in family government, church government, and civil government.
This has been held by establishmentarians (theocrats or ‘state church’ advocates like Luther, Calvin, and most of the early Reformers), but isn’t necessarily tied to that. (It should be understood that establishmentarianism is an inescapable category that all men embrace.)
A potential problem here is when family, church, or civil governments are ruling as mediate stand-ins for the authority of Christ but are ruling inconsistent with the authority they are intended to be representing (Christ’s authority). A visible/material mediate authority that does not mediate Christ’s rule is an authority (Kingdom) that is no kingdom that needs to be recognized by God’s kingdom people who desire that their magistrates in family, church, and civil rule consistently with the revelation of God.
R2K begins to creep in here when it absolutizes these disobedient visible kingdoms and gives them a mediate authority that is sequestered and cordoned off from being informed by God’s word. R2K absolutizes these visible kingdoms and insists that they should not be ruled by God’s special revelation appealing instead that the mediate realms are ruled by the ever amorphous Natural Law.
3.) Here we find the distinction between the external mediated Kingdom of the civil realm (civil government) and the kingdom of the institutional Church. This two-kingdom theology finds that in the Church Christ is the one head and king and rules explicitly through His Word as ministered by duly ordained church officers, meanwhile in the civil Government rule not by Christ’s Word but by the sword.
Of course, a potential problem here is that civil Government has to have some standard by which the sword is brought to bear. R2K desires that standard to be the ever shape-shifting Natural Law, while Biblical Christians desire that standard be God’s Law Word as found or legitimately deduced from Scripture.
One advantage of this 2K model when properly used is that it keeps the snout of civil magistrates out of the affairs and rulings of the institutional Church. This 2K model insists that a civil realm magistrate as he enters into the church realm leaves behind his status and is just one more member of the kingdom wherein Christ is ruling in the Church. The magistrate comes into the Church realm under Christ’s officers in the Church. Civil magistrates have no authority in the church realm any more than any other member of the Church. Throughout history, would-be tyrants in Christian lands have hated the restriction this 2K model brings.
4.) Here we begin to creep into clear R2K territory, at least as R2K as bastardized this concept. The fourth version of 2K theology sometimes co-exists with any of the above views. It teaches that there is a kingdom of “common grace,” and a kingdom of “special grace.” In this concept, one kingdom exists where people of all different faiths live, move and have their being as they live various aspects of life that are common to all people. This is the kingdom of common grace. The other kingdom in this model would be that kingdom wherein Christians as Christians alone live and move and have their being. Obviously, this would include the Christian’s life in the Church and has particular reference to Word & Sacrament.
This one is especially where R2K begins to bollix things up. It typically so desires to live the hyphenated-life where there exists an imagined impermeable wall between the kingdom of common grace and the kingdom of special grace that no distinctly Christian category can any longer exist in the common realm. For R2K in this model Christian education, Christian law, Christian family, Christian culture, Christian kings, Christian art can’t be conceived of since education, law, family, culture, kings, art, etc. by definition are only common and therefore cannot be handled as being distinctly Christian.
Here it is not only rabid amillennialism that is playing into matters but the whole idea of common grace has been let loose from any Christian mooring and is allowed to play havoc with the idea that Christ has all explicit authority in heaven and on earth.
5.) There is a 2K version wherein the now, not yet hermeneutic is employed. This 2K version would speak of the now present inaugurated kingdom of grace and the not yet but certain future consummated kingdom of glory. This view is consistent with any number of the above views and is hardly contested except among die-hard premills who see the kingdom as all future and die-hard full Preterists who see the kingdom as all present.
I am indebted to this post for my expanding work here.