A common and simple outline of church history says that the purity of the early Church was lost during the era when Constantine was the emperor of Rome. Between the end of the Constantinian era and the Reformation, the Middle Ages muddled through the corruption of the Church with small groups of monks who preserved whatever purity of the early Church that was left. The Reformers recovered the purity of doctrine but not Christian discipleship or conduct because they were too closely allied with a National Church ( the Constantinian problem all over again). In Modern Times the Church is splintered, shallow and in some places serves to prop up a nationalistic idea of the state, ie, American civil religion or Dutch Reformed support of the state in South Africa. There are pockets of the "purer" version of early Christianity in places and some Churches seek to recover that purity but generally the Church today is in a "fallen" shape due to the Constantinian compromise. That compromise allowed the Church to exist freely as long as it supported the state. Before Constantine, Christians were known by their lifestyle because it was risky to be a Christian. After Constantine, there was no risk, everyone was just a Christian. Today, the Church is full of "just Christians" and no one really knows what it means to be a Christian. But, it doesn't mean much. Certain Anabaptists will say that the Reformation did not go far enough - it did not reform discipleship/conduct - and it did not overturn the Constantinian compromise. The purity of the early Church has been lost and so we are members of a "fallen" church today. John Howard Yoder is the main proponent of this view.
Peter Leithart has issues with this common, simple outline of Church history and with Yoder's analysis of the Constantinian problem. His book, Defending Constantine, is a richly researched history of Constantine and his era as well as a study of the theological and political implications of that history. It seems to me Leithart has shown that Constantine was a genuine Christian who had a positive impact on the history of the Church ( the common, simple outline suggests his impact was mostly negative). The generally accepted view of Constantine is cynical. He used the Church to further his political vision of a unified and ever expanding empire. And as he gave the Church privileged status, the Church reciprocated by supporting his vision for empire. That's the cynical view and Leithart has shown it to be wrong.
Leithart shows that Constantine was a man of simple but true faith. He was also a ruler and a military man, an emperor of his times. He tried to practice his Christian faith as best he could. He ended the widespread persecution of Christians which I assume most of them were grateful for. Although his policies favored the Church, he allowed for religious freedom. He did not have a policy of forced conversion and pagans retained high positions in his administration. He built many Church buildings and supported Church ministries of compassion and mercy. He appointed numerous Christians to leadership positions in his government but he did not attempt to Christianize the legal system. One of his acts that had the most far reaching implications was his decision to end the sacrificial system of Rome. As Leithart says, "Roman sacrifice was at the center of Roman civilization." "It was the chief religious act by which Romans communicated and communed with the gods, keeping the gods happy so Romans could be happy." Roman senators sacrificed when they made deals and decisions. Soldiers sacrificed to their gods for success on the battlefields. Citizens were required to show their devotion to the Roman gods and the emperor by sacrifice. Christians refused to sacrifice to Roman gods and so prior to Constantine they were sacrificed. Roman society demanded sacrifice. When Constantine eliminated sacrifice, Leithart says, he unintentionally sowed the seeds of Rome's eventual collapse, "and established that Rome's life now depended on its adherence to another civic center, the Church." The Church is based on Christ's sacrifice for us and he calls us to live a sacrificial lifestyle. Christ's sacrifice started a new city whose citizens live by mutual love and service. What Constantine probably did not see was that he was not just initiating a new religion ( cultus) into the Roman mix of religions, but he was welcoming another city (polis), the city of God, Christ's city, the body of Christ which cannot be co-opted by any human powers.
So in Leithart's view we can be hopeful about the Church's future. In his view, the Church never did "fall" in the Constantinian era. He sees a much more resilient Church than Yoder does. The Church is the model community of justice and peace that other political leaders should imitate. It is God's alternative polis. There never was a pure Church so there is no point trying to find it and get back to it. In every age, as the times and eras change, God has worked by His Spirit in his Church to show the world what a true city looks like and what the City of God will look like some day.
Modern politics does not welcome the Church, the true city. Modern states are happy to use the Church as long as it knows its place. It needs to remain out of the public sphere and focus on piety and personal faith, propping up the state and it's causes when asked. Modern states denounce the Constantinian system. Totalitarian states sacrifice Christians all over again. Democratic states marginalize the Church and only accept it as a cheerleader for its causes. However, all modern states depend on sacrifice too. Unwilling to accept the final sacrifice of Christ, they continue to offer human sacrifices through various pogroms and wars.
Leithart agrees with some of the radicalism of Yoder but he also counsels patience. Just as Rome wasn't built in a day, neither is the city of God. But it is coming and it can be seen and experienced in part, in many places in the world today. The Church is not either pure or apostate which are the only two options Yoder seems to recognize. There are are a number of other points on the spectrum. There have been historically and there are today. Augustine said that in "this middle time" between Christ first advent and the coming kingdom of God we must always pray, forgive us for our sins. Yoder and others don't think we can expect much in this middle time. But, Leithart says, not so. Through reevangelization, and forming a Christ centered politics and a fresh public confession that Jesus's city is the model city, his blood the only expiating blood, his sacrifice the sacrifice that ends sacrifice, we can witness to the City of God amid the cities of earth.