Secularism in the modern political meaning – the idea that religion and political authority, church and state are different, and can or should be separated – is, in a profound sense, Christian. Its origins may be traced in the teaching of Christ, confirmed by the experience of the first Christians; its later development was shaped and, in a sense, imposed by the subsequent history of Christendom. The persecutions endured by the early church made it clear that a separation between the two was possible; the persecutions inflicted by later churches persuaded many Christians that such a separation was necessary.
Bernard Lewis, What Went Wrong? (2002), 96–97 (HT: Darryl Hart)