The eleventh and twelfth centuries saw an expansion of Christianity into Scandinavia, the Baltic lands, eastern Europe, and Spain that had profound cultural consequences. The expansion was accomplished through wars, the establishment of new bishoprics, and the vast migration of Christian colonists into non-Christian territories. More and more Europeans began to think of themselves as belonging to a realm of Christianity that was political as well as religious, a realm they called Christendom.

Christian influences entered Scandinavia and the Baltic lands primarily through the creation of dioceses (church districts headed by bishops). This took place in Denmark and Norway in the tenth and eleventh centuries, and then in Sweden and Finland. In all of these areas, Christian missionaries preached, baptized, and built churches. Royal power advanced institutional Christianity, and traditional Norse religions practiced by the Vikings were outlawed. In eastern Europe the German emperor Otto I (see “The Restoration of Order”) planted a string of dioceses along his northern and eastern frontiers, hoping to pacify the newly conquered Slavs in eastern Europe. German nobles built castles and ruthlessly crushed revolts by Slavic peoples.

The church also moved into central Europe, first into Bohemia in the tenth century and from there into Poland and Hungary in the eleventh century. In the twelfth and thirteenth centuries thousands of settlers poured into eastern Europe from the west.

The Iberian Peninsula was another area of Christian expansion. In about 950 Caliph Abd al-Rahman III (912–961) of the Umayyad Dynasty of Córdoba ruled most of the peninsula. Christian Spain consisted of a number of small kingdoms. In the eleventh century divisions and civil wars in the caliphate of Córdoba allowed Christian armies to conquer an increasingly large part of the Iberian Peninsula. By 1248 Christians held all of the peninsula save for the small state of Granada in the south.

Fourteenth-century clerical writers would call the movement to expel the Muslims the reconquista (ray-kon-KEES-tah; reconquest) — a sacred and patriotic crusade to wrest the country from “alien” Muslim hands. This religious idea became part of Spanish political culture and of the national psychology. Rulers of the Christian kingdoms of Spain increasingly passed legislation discriminating against Muslims and Jews living under Christian rule, and they attempted to exclude anyone from the nobility who could not prove “purity of blood” — that is, that they had no Muslim or Jewish ancestors.

Spain was not the only place in Europe where “blood” became a way of understanding differences among people and a basis for discriminatory laws. When Germans moved into eastern Europe and English forces took over much of Ireland, they increasingly barred local people from access to legal courts and denied them positions in monasteries or craft guilds. They banned intermarriage between ethnic groups in an attempt to maintain ethnic purity, even though everyone was Christian.