The basic distinction, which really is too basic and needs clarification, is that Christ reigns over the church through the conscience and sacraments while the magistrate orders society in accordance with external matters—in light of nature and (ideally) Scriptural revelation.
Such a view prevents one from minmizing the reign of Christ, as if Christ only reigns in the church and not society since not everyone is yet converted. But if Christ only reigns over the converts and not creation, he is not the Lord of Creation.
But Scripture says God reigns over Babylon, Assyria, Egypt, and the world. To minimize his reign by arguing that Christ only reigns when a society as a whole converts to Christianity in effect undercuts the present universal reign of Christ.
But in fact, he does reign and when we returns all will recognize that reign visibly and publically when every knee will bow.