The Bible's many books offer a variety of options. Scripture written in good times suggests that good times begin the reward: corn and wine and oil increase, and they are a down-payment on being faithful to God. Scripture written in bad times argues that bad times make good Christians: in fact they are a necessary entrance pass. "We are God's heirs and Christ's fellow-heirs, if we share his sufferings now in order to share his splendour hereafter," Paul told Christians in Rome. Unpleasant phenomena like being thrown to lions by Nero were included in the deal.
By contrast, during the 1,000 years covered by Richard Fletcher's masterly book, most of those lining up for baptism expected immediate benefits. The trend started with the Roman Emperor Constantine I, who believed that he won the most important battle of his life by direct decisions of the God of the Christians. He does not seem to have had any further instruction in Christian doctrine. He poured out money and favours on the Church. This was a fateful turning-point in its history.
When Fletcher's story opens, Constantine's successors were turning the alliance between Church and State into permanent establishment. Christianity and the Roman Empire were now inseparable. The Church called itself Catholic, which means worldwide, but its world was that of the Roman state - urban, suave, tidy-minded - founded long before Jesus Christ lived in backwoods rural Palestine.
This had a curious effect when the western, Latin half of the Roman Empire fell to pieces in the fifth century. The Latin-speaking Church became a curator of Romanness. That was a paradox, since Jesus had been crucified by a Roman governor, but the alliance stuck. Bishops still dress up on sacred occasions in copes and mitres, a version of late Roman aristocrats' best clothes. Monks who began by opting out of Roman society, as Paul had urged, took to copying classical manuscripts. Without these monks, very little would survive of Greek and Roman literature; it would have crumbled to dust.
By Fletcher's closing date of 1386, Christianity conquered all Europe. In that year even the highly sophisticated pagans of Lithuania, making the best of a bad job, allied with one Christian power to avoid annihilation by another. Why the success? In the previous 1,000 years, a mirage of the Roman Empire haunted the peoples who had helped to demolish it: Goths, Franks, Saxons. They wanted to be Roman, and the Bishop of Rome was happy to oblige. When he sent a mission to the English in 597, he turned Kent into a little Italy, with churches and cathedrals dedicated in the same way as the leading churches in Rome. An Anglo-Saxon king even retired to Italy, thus inventing Chiantishire.
In 800, Christian Europe carried its love affair with dead Rome to the extreme of inventing a new monarch who called himself the Holy Roman Emperor. Like Constantine, many such rulers saw Christianity as a religion which won battles. They were also prepared to send in the troops to save souls. Not all missionaries were happy about this, but they remembered how Augustine of Hippo had wrestled with the ethics of forcible conversion. He pointed out that Jesus had told a parable in which the host of a wedding party filled the room by getting his servants to force people to come along. "Compel them to come in" became a missionary slogan: "benignant asperity", which means clobbering people with the best of intentions. The Crusaders took this to its logical conclusion by gathering armies to fight (and massacre) non-Christians. It was a long way from turning the other cheek.
So there were carrots and sticks in converting medieval Europe. Few seem to have understood conversion as Billy Graham might today. Most people were ordered to become Christians, usually by their lord or lady. But it was not all mindless coercion. The Church could be sensitive to the pride of the people, and one of Fletcher's major themes is the way it married new to old.
In many places, it allowed people to go on expressing their grief by filling the graves of the dead with prized possessions. Even the great Christian holy man Cuthbert of Lindisfarne was given his grave goods to take with him. The Church encouraged royal families to trace their genealogy further beyond the fierce pagan God Woden, all the way back to Biblical Adam. Bishops outshone non-Christian religious leaders with their splendid hospitality. Wilfrid of York threw a three-day party for high society after dedicating what is now Ripon Cathedral. No doubt the occasion was a satisfying mixture of solid Anglo-Saxon cheer and delicate Roman canapes, if anyone was capable of remembering afterwards.
Fletcher writes deliberately for the non- specialist. He avoids false piety, and effectively conveys the sheer strangeness of the Christian faith in past contexts. Even if God exists, She or He needs constantly to be reinvented, and this is a lively panorama of some of the reinventions fuelling a millennium of Christendom.