A History of Christianity by Diarmaid MacCulloch
Greco-Roman religion had ended up with an uneasy mixture of the cult of the emperor (increasingly odd as the empire became a military dictatorship constantly changing hands after bloody conflicts) and a chaotic plurality of local rites and myths. The Jewish world was marked by a lively tension over how Jewish identity was to be understood. What Christianity brought into all this was a definition of Jewish identity that opened up to become a definition of human identity independent of any particular state apparatus; it created, you could say, the very idea of a religion as a form of belonging together that did not depend on political loyalties.
Of course, Christians rapidly worked out how to deploy political power and to enforce conformity. But MacCulloch resists the glib narrative of decline and fall which is always going to tempt the sceptical historian of the church. Instead, he traces the sheer variety of ways in which the basic forms of Christian life and faith were fleshed out. As a serious historian, he brushes aside the luxuriant growths of conspiracy theory – the Gnostics plus Mary Magdalene plus Knights Templar fantasy world. But he also cautions against the popular current assumption that minorities and dissidents in past ages were enlightened moderns in disguise – reminding us, for example, that Pelagius’s opposition to Augustine on original sin was not a sunny and optimistic vision but part of a fiercely rigorous morality that left little room for the lights and shadows of human experience and the uneven quality of what we call freedom.