Since it is Christmas time, the celebration of two important pagan festivals appropriated by Christianity, it is interesting to read Bernard Cornwells’s article at Omnivoracious on how Christianity spread through Europe.
One answer is that Christianity proved more profitable. There is a telling story about King Edwin of Northumbria, a powerful pagan who ruled what is now northern England and southern Scotland in the 7th Century. He probably worshipped the Norse gods like Thor and Woden, but at some point he encountered a Christian missionary who suggested that success in war and material prosperity would follow a conversion. Edwin put that to the test and god came through with a battlefield triumph and massive amounts of plunder. The king’s chief pagan priest told Edwin that the old gods had never shown such favor and that Northumbria should therefore convert, which it duly did. The story echoes the experience of Constantine, the Roman Emperor who converted because the Christian god gave him victory over Maxentius. It is a common enough tale. In the early 10th Century a Viking named Hrolf took land in what is now Normandy and the treaty confirming his possession insisted he became a Christian. ‘Paris,’ Henry IV of France declared when he changed from Protestant to Catholic, ‘is worth a mass.’ The Duchy of Normandy (which led to the throne of England) proved well worth a mass too.[Bernard Cornwell, Author of “The Pagan Lord,” Muses on the Path to Christianity]