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An accident of political development has made it possible to divide the Christian world, in the first place, into two great halves, Eastern and Western.
The root of this division is, roughly and broadly speaking, the division of the Roman Empire made first by Diocletian (284-305), and again by the sons of Theodosius I (Arcadius in the East, 395-408; and Honorius in the West, 395-423), then finally made permanent by the establishment of a rival empire in the West (Charlemagne, 800).
To find a time when there was one Eastern Church we must go back to the centuries before the Council of Ephesus (431).
Since that council there have been separate schismatical Eastern Churches whose number has grown steadily down to our own time.
Whereas our Latin Fathers have never concerned them at all (most Eastern Christians have never even heard of our schoolmen or canonists), they still feel the influence of the Greek Fathers, their theology is still concerned about controversies carried on originally in Greek and settled by Greek synods. Further legislation formed two more at the expense of Antioch: Constantinople in 381 and Jerusalem in 451.
The literature of those that do not use Greek is formed on Greek models, is full of words carefully chosen or composed to correspond to some technical Greek distinction, then, in the broadest terms, is: that a Western Church is one originally dependent on Rome, whose traditions are Latin; an Eastern Church looks rather to Constantinople (either as a friend or an enemy) and inherits Greek ideas. In any case the Roman patriarchate was always enormously the greatest.
Outwardly, the bond of a common language and common liturgy is often the essential and radical division of a schism.
It is now possible to draw up the list of bodies that answer to our definition.
Historically and archeologically, it is a secondary question.
Each Catholic body has been formed from one of the schismatical ones; their organizations are comparatively late, dating in most cases from the sixteenth and seventeenth centuries.
Looking at a map, we see that, roughly, the division between the Roman patriarchate and the others forms a line that runs down somewhat to the east of the River Vistula (Poland is Latin), then comes back above the Danube, to continue down the Adriatic Sea, and finally divides Africa west of Egypt.
Illyricum (Macedonia and Greece) once belonged to the Roman patriarchate, and Greater Greece (Southern Italy and Sicily) was intermittently Byzantine.