In a review essay of Wayne Grudem’s Politics According to the Bible: A Comprehensive Resource for Understanding Modern Political Issues in Light of Scripture and Peter J. Leithart’s Defending Constantine: The Twilight of an Empire and the Dawn of Christendom, Darryl Hart writes:
. . . the New Testament, Christ, and the apostles offer no prescriptions about civil polity other than that believers should submit to the established authorities. They did prescribe a spiritual polity—the church—and in that order the sacrifices of both the Jewish and pagan peoples came to an end. But the Christian church was an adaptable institution that could exist—even despite persecution—in a variety of political orders, whether a pagan empire or a federal republic. In which case, both books assume what they do not prove. Each author in his own way suggests that for a kingdom as substantial as Christ’s to be great, it must have a civil expression. Leithart even says in his very last sentence that if modern civilization is to avoid “apocalypse” it must also come forward to be baptized the way Rome was.
In point of fact, Constantine’s baptism of Rome could not prevent the empire’s fall any more than restoring a Christian influence in the United States will avert the American republic’s demise. Christ died for his church, not for political orders. The only way for civilians to avoid judgment is not through the policies of Christian rulers but by being baptized and joining the body of Christ. No matter where such Christians live, no matter what the political order to which they submit, their future is secure if they belong to the king who is also a prophet and a priest.
– “To Render unto or Baptize Caesar: Is that the Question?” (Ordained Servant Online)