In 312 the Emperor Constantine declared Christianity the religion of the Roman Empire. Thus began the ‘Constantinian’ or ‘Christendom’ era in the history of Christianity. After nearly three centuries of intermittent, but sometimes very severe, persecution by the civil authorities, Christianity was now in alliance with the powers that be. By the end of the fourth century a large part of the Roman elite had converted to Christianity and other forms of religious worship were prohibited. The process by which the mass of the population became fully integrated into the Christian church was much more long drawn out. Equally, a variety of rival religions continued to be privately practised long after they were officially proscribed. But a pattern of relations between church and state and between church and society had been established. It would be repeated as Christianity spread to northern and eastern Europe and, much later, to the Americas. For the next 1500 years most Christians learnt and practised their faith in the context of ‘Christendom’. That is, they lived in a society where there were close ties between the leaders of the church and those in positions of secular power, where the laws purported to be based on Christian principles, and where, apart from certain clearly defined outsider communities, every member of the society was assumed to be a Christian.

Naturally ‘Christendom’ has been challenged by non-Christians, whether in fourth-century Rome or in modern Europe. But it has also been a subject of intense debate between Christians.