Christianity crossed this sea in both directions and more than once. Irish monasticism reached the Scottish islands at Iona in the sixth century and northern England at Lindisfarne in the seventh; from Northumbria the traffic reversed, and Willibrord sailed to Frisia in 690 and Boniface into Frisia and Saxony, where he was killed in 754. The manuscripts produced along this coast, the Lindisfarne Gospels among them, are the art of a single cultural province rather than of separate countries.
In 793 the direction changed again. The raid on Lindisfarne, recorded with horror in the Anglo-Saxon Chronicle, opened three centuries in which the same searoads carried Scandinavian fleets, settlement and eventually the conversion of Scandinavia itself, so that by the eleventh century the whole rim was Christian.
Michael Pye's history argues that the North Sea world, not the Mediterranean, made much of what is characteristic about northern Europe, from its law and money to its habits of literacy. The Reformation moved on the same water: Tyndale's English New Testament was printed in the Low Countries and smuggled across in bales of cloth, and the English separatists who sailed from Delfshaven in 1620 had already been living in Leiden for a decade. The fishing communities kept their own religion of the sea, with fishermen's missions and blessings of the fleet that continue in ports on both shores.