The Crusades had a profound impact on both Europe and the Middle East, but they were not the only example of Christian expansion in the High Middle Ages. As we saw earlier, Christian kingdoms were established in the Iberian Peninsula through the reconquista. This gradual Christian advance was replicated in northern and eastern Europe in the centuries after 1000. People and ideas moved from western France and western Germany into Ireland, Scandinavia, the Baltic lands, and eastern Europe, with significant consequences for those territories.

Ireland had been Christian since the days of Saint Patrick (see "Missionaries' Actions" in Chapter 7), but in the twelfth century, Norman knights crossed from England, defeated Irish lords, and established bishoprics with defined territorial dioceses. Latin Christian influences also entered the Scandinavian and Baltic regions primarily through the establishment of dioceses. Otto I established the first Scandinavian dioceses in Denmark. In Norway, Christianity spread in coastal areas beginning in the tenth century, and King Olaf II (r. 1015–1028) brought in clergy and bishops from England and Germany to establish the church more firmly. From Norway, Christianity spread to Iceland; from Denmark, it spread to Sweden and Finland. In all of these areas, royal power advanced institutional Christianity, and traditional Norse religions practiced by the Vikings were outlawed.

In eastern Europe, the German emperor Otto I planted a string of dioceses along his northern and eastern frontiers, hoping to pacify the newly conquered Slavs. German nobles built castles and ruthlessly crushed revolts by Slavic peoples, sometimes using the language of crusade to describe their actions. A military order of German knights founded in Palestine, the Teutonic (too-TAH-nihk) Knights, moved their operations to eastern Europe and waged wars against the pagan Prussians in the Baltic region. After 1230, from a base in Poland, they established a new Christian territory, Prussia, and gradually the entire eastern shore of the Baltic came under their hegemony.

The church also moved into central Europe, first in Bohemia in the tenth century and from there into Poland and Hungary in the eleventh. In the twelfth and thirteenth centuries, thousands of settlers poured into eastern Europe. These immigrants were German in descent, name, language, and law.