Within the Persian Empire, the main Christian church was based in the twin cities of Seleucia-Ctesiphon, the imperial capital that was the successor to ancient Babylon, and the most populous city in the world at that time. This church followed the teachings of Nestorius after 431, detaching itself from the authority of Antioch: in 498 its head, the catholicos, took the title of patriarch of Babylon, the patriarch of the East.
