The first step in trying to shape the future is to understand the past, and Stuart Murray’s volume Post-Christendom uses many chapters to detail the history of how the church became dominant in Western Europe.

He begins by taking us back to the fourth century AD when an emperor from the eastern half of the Roman Empire, Constantine, supposedly became a Christian in 312 as he was attempting to wrest control of the whole empire. He succeeded in his quest and was sole emperor until his...