The New Zealand Anglican priest Glynn Cardy has written an excellent essay in his blog on the need for separation of church and state. He correctly points out that "Christianity does not need preferential treatment". But he goes on to make a further, deeply insightful comment. He rightly notes that not only does Christianity not need a privileged status in government, but in fact such a cozy relationship with the ruling forces of society damages the very core of the Christian faith--which is, after all, very much about identifying with the powerless, as Jesus himself did. And this is simply not possible if Christianity makes an unholy alliance with the powerful. As he puts it:
In the history of Christianity the relationship with governments and ruling powers has been a mixed blessing. When a government adopts a religion as it’s favoured or only brand inevitably that religion is forced to make compromises. The ability of the Christian faith, for example, to critique regal injustice has often been compromised by the royal privileges it’s been consuming. For a religion to have power and wealth does not necessarily mean that faith will flourish, often the reverse.It has fascinated me for some time how early Christianity abandoned its initially radical message of Jesus against the power of the Roman Empire, and thus lost its moral independence from the state. The book When Jesus Became God by Richard Rubenstein provides a fascinating description of the dissolution of this Christian independence from the state during the fourth century CE. While the book is nominally about the theological battles that took place between the Nicene and Arian versions of the faith during that century, the backdrop is the relationship between church and state that developed also during that time. One thing becomes abundantly clear from that book--the faction fights within the church did not take place independently of the internal political struggles of the Roman Empire. Of course, it is well known that Constantine involved himself deeply in this theological dispute, and in fact it was he who played the host at Nicea, where he then threatened with exile anyone in attendance who did not assent to the creed that was developed there. But the dispute over Arianism versus Niceanism did not end with Constantine. At various later points his successors duked it out over this issue; for example, Valens, who ruled in the East, was an Arian, while Valentinian, who ruled in the West, was a Nicean. Ultimately, when the Nicean version of Christianity triumphed, the state involved itself directly in the dispute by making Arianism a capital offense. Thus the diversity that had existed within Christianity for centuries was officially stamped out in favor of a homogeneous "catholic" religion by the Roman state.
Separation of church and state is not an atheistic idea dreamt up by those who wanted religion marginalized. It was an idea actively promoted by Christians who heeded Jesus’ warnings about the corrosive nature of power and wealth, and who were familiar with their history. Getting into bed with governments is not good for Christianity. Having Christianity with favoured status would also provide a precedent for the future when another religion might oust Christianity from its place of privilege. States with embedded religion usually oppress religious minorities.
Of course, the relationship between church and state during that time was not simply a one sided involvement of state power in suppressing freedom of theological thought. The church, by making this unholy alliance with the state, lost its moral credibility as a critic of the tools and methods of oppressive power; and it certainly could not oppose the very existence of that power, as Jesus did in his own nonviolent way.
Once the church and state form an alliance, the church can no longer act as moral conscience against the abuses of power that inevitably happen by the ruling classes of society.
Amazingly, despite centuries collusion with state power by Christianity, the message of Jesus and of his Jewish prophetic forebears manages to break through the walls of dogma and hierarchy. From Quakers who opposed slavery in the United States in the first half of the 19th century, to liberation theologians in Latin America who promoted a preferential option for the poor in the late 20th century, there have always been those within the Christian traditions who found themselves called by God to speak truth to power, just as Jesus did two millenia ago.