Presiding over these far-flung outposts from his seat in the Mesopotamian city of Seleucia (near what is now Baghdad, Iraq) was the Catholicos, the patriarch of the Church of the East. The most prominent of these patriarchs was Timothy I, who held the position from 780 to 823.
To appreciate the scale of his domain, consider that by the early ninth century England had two Catholic bishops, one in York and one in Canterbury. At the same point in history, Timothy presided over nineteen metropolitans and eighty-five bishops, including a bishop in the rugged mountains of Tibet.