The story of Christianity in eastern Europe and northern Eurasia is complex—a tangled web of changing peoples, nations, and church allegiances; of political, military, and cultural conflicts; and of ideological, social, and spiritual forces in a seemingly perpetual flux. This article traces the course of twenty centuries of Christian history in this region, which is bounded on the south by the tip of the Greek Peninsula, ringed roughly by the Adriatic, Aegean, Black, and Caspian Seas; on the north by the Baltic Sea and the Finnish Peninsula; on the east by the Ural Mountains; and on the west by the eastern slopes of the Alps and the river Elbe.

The history of the Christian Church in eastern Europe and northern Eurasia can be understood through the interplay over the centuries of four major factors: Greek-Byzantine, Latin-Roman, and Frankish-German influences, and the...