Christianity is the most enduring and influential legacy of the ancient world, and its emergence the single most transformative development in Western history. Even the increasing number in the West today who have abandoned the faith of their forebears, and dismiss all religion as pointless superstition, remain recognisably its heirs. Seen close-up, the division between a sceptic and a believer may seem unbridgeable. Widen the focus, though, and Christianity's enduring impact upon the West can be seen in the emergence of much that has traditionally been cast as its nemesis: in science, in secularism, and yes, even in atheism. Ranging in time from the Persian invasion of Greece in 480 BC to the on-going migration crisis in Europe today, and from Nebuchadnezzar to the Beatles, the historian explores just what it was that made Christianity so revolutionary and disruptive; how completely it came to saturate the mind-set of Latin Christendom; and why, in a West that has become increasingly doubtful of religion's claims, so many of its instincts remain irredeemably Christian.