Yoder makes the point this way: prior to Constantine, Christians believed that God was active in the church, but they had to have faith that God was active in the world, since the world was beating the hell out of them. After Constantine, Christians had to have faith that God was active in the church because they now knew that God was active in the world, running the world through Caesar.
Another way to put it: before Constantine it took courage to be a Christian; after Constantine it took courage to be a pagan.
\~ Stanley Hauerwas, "Dietrich Bonhoeffer on Truth & Politics"
In the early church there was relatively little penetration of Christians in the centers of terrestrial power. As time went on, Christians gained influence in the Roman empire. This culminated in the conversion of Emperor Constantine, which launched a new epoch in the church. Now rather than being a sometimes-persecuted minority, Christians were running the largest empire on the planet. This meant that they had the opportunity to enshrine their values into law, and godly living was merged with responsible civics.
I draw a comparison between Constantinianism and the Kingdom of Israel. God's chosen nation had received its legal code via Moses the prophet. Through the Exodus, the wandering, the settlement of Canaan, the period of the judges, and finally in the monarchy, the Torah was the law of the land. In this way Israel was similar to the Roman Empire and what followed: the marriage of the church with the state. Of course Israel's life under the law of Moses was never perfect, and neither was the church's. Yet the basic framework ensured that people would feel at home in their country with respect to their religion.
The kingdom of Israel eventually split and was in the North dispersed by the Assyrians and in the South carried into exile by the Babylonians. This marked the end of the rule of Torah in a nation state. Jews had to transition from being at home in a nation which embraced their religion to being sojourners in a foreign land.
The end of Constantinianism, however, is not so clearly demarcated. In my lay analysis, the first cracks started appearing during the Radical Reformation, when church first started splitting from the state. The founding of the United States, with the explicit absence of a national church, was an important waypoint. However, no matter the impetus of religious freedom which brought so many immigrants to these shores, the generic "Christian" character of society persisted in the absence of a national church. Christians still ran the show and set policies which largely reflected their understanding of the scriptures and tradition.
And in this present day we have people decrying the loss of traditional values and the influence of Christianity in the public square. Future historians will be better equipped to make this determination, but I suggest that the end is nigh. When Constantinanism ends - when Christians are no longer running the show politically - it will be the beginning of our Babylonian exile.
I think many young Christians in this country have already adopted the attitude of sojourners. This is why two generations which largely share the same values sourced in Christianity can have such different opinions on politics. Young people are less likely to agree that Christian social values should be legislated. Why? Because we don't own the world, and we know that law cannot really change hearts (think of the Three Hebrew Children in the Fiery Furnace).
But shouldn't we Christians fight to retain control of the culture and politics of this country? No. The problem with the culture war is the metaphor itself. Jesus taught that our fundamental relationship with the world is one of salt and light. The light is the positive example we set by our own conduct (not born of our fear of the state, but of our affection for God). Salt is the preservative for when society does not heed the example of the light. Neither is coercive. It is in being salt and light that we Christians, like the Jews sent into exile in Babylon, seek the peace and prosperity of the city in which we are sojourning.
The end of Constantinianism requires Christians to be courageous once more. But we should not fear, since the examples of Jesus and the apostles were forged in a period when they had no influence over the politics of Rome or even the Sanhedrin. We do not need to win political contests in order to carry out the mission of being salt and light to our society. I will argue moving forward that bearing witness to truth and providing comfort to our broken neighbors is a more effective politics than winning elections anyway.