The line that separated Eastern Christendom from Western on the medieval map is similar to the "iron curtain" of recent times. Linguistic barriers, political divisions, and liturgical differences combined to isolate the two cultures from each other. Except for such episodes as the schism between East and West or the Crusades, the development of non-Western Christendom has been largely ignored by church historians. In The Spirit of Eastern Christendom , Jaroslav Pelikan explains the divisions between Eastern and Western Christendom, and identifies and describes the development of the distinctive forms taken by Christian doctrine in its Greek, Syriac, and early Slavic expression.

"It is a pleasure to salute this masterpiece of exposition. . . . The book flows like a great river, slipping easily past landscapes of the utmost diversity—the great Christological controversies of the seventh century, the debate on icons in the eighth and ninth, attitudes to Jews, to Muslims, to the dualistic heresies of the high Middle Ages, to the post-Reformation churches of Western Europe. . . . His book succeeds in being a study of the Eastern Christian religion as a whole."—Peter Brown and Sabine MacCormack, New York Review of Books