The historical transition from the Pagan Roman Empire to the Christian Roman Empire was a big part of Christian history, having massive implications. How should it be viewed? Here are some thoughts.
Though it is difficult (for me) to affirm any extreme theological assertion with regards to many matters, in my opinion, Christianity cannot (ever) provide an authentic basis for political rule. The only alternative, biblically speaking, is to live in a (paradoxical) way that seeks to acknowledge Christ as the King above all kings, while living and engaging with a world that does not. (As apposed to running away from it, or inventing doctrines that promise future escape, completely disregarding the creation narrative... but that's another issue).
At the fall of Rome, pagan's argued that Rome’s traditional gods were wreaking revenge and causing its downfall for siding with the God of the Christians. Augustine responds by differentiating God’s kingdom from the kingdom of the earth: The "Christian" kingdom of Rome was not the Kingdom of God on earth.
Jesus’ claim that his kingdom is not of this world does not mean that it does not effect this world, on the contrary, it is one that subverts the worldly kingdom with a Kingdom in which the “first shall become last, and the last shall become first” (Mark 10:31). I agree with Jacque Ellul that the militant and political interpretation of the Gospel is a falsification, and I would add that it is a recurring misunderstanding of the Gospel message, revealed with Jesus’ own disciples: their expectations of what was to happen represent the same expectations of all of Israel; that God would eventually send a Messiah who would militaristically and politically take charge of Israel (Luke 9:46; Mark 9:33; Matt 18:1; Acts 1:6).
The alternative then, is to seek the kingdom of God as revealed in the Gospels: a kingdom that subverts all notions of unequal and unjust social systems through selfless love: a world in which claims to rank and status have no place at all. This is a paradigm exemplified most beautifully by God himself, who leaves his throne to die on a cross in shame.
How should this impact our political stance?