At least one East Syriac diocese in Khorasan existed by the beginning of the fifth century, though it was not assigned to a metropolitan province in 410.
After establishing five metropolitan provinces in Mesopotamia, Canon XXI of the synod of Isaac provided that 'the bishops of the more remote dioceses of Fars, of the Islands, of Beth Madaye, of Beth Raziqaye and of the country of Abrashahr must accept the definition established in this council at a later date'.
The bishops Bar Shaba of Merv, David of Abrashahr, Yazdoï of Herat and Aphrid of Segestan were present at the synod of Dadishoʿ in 424.
The growing importance of the Merv region for the Church of the East is also attested by the appearance of several more Christian centres during the late fifth and sixth century.
The bishops Yohannan of 'Abiward and Shahr Peroz' and Theodore of Merw-i Rud accepted the acts of the synod of Joseph in 554, the latter by letter, while the bishops Habib of Pusang and Gabriel of 'Badisi and Qadistan' adhered by proxy to the decisions of the synod of Ishoʿyahb I in 585, sending deacons to represent them.
The surviving urban Christian communities in Khorasan suffered a heavy blow at the start of the thirteenth century, when the cities of Merv, Nishapur and Herat were stormed by Genghis Khan in 1220.
[9] The priest Maraq was among the signatories of the acts of the synod of Ishoʿyahb I in 585, on behalf of the metropolitan Gregory of Merv.
[15] The priest Daniel was among the signatories of the acts of the synod of Ishoʿyahb I in 585, on behalf of the metropolitan Gabriel of Herat.
[18] In 540 the diocese of Segestan was disputed between two bishops, Yazdaphrid and Sargis, consecrated during the schism of Narsaï and Elishaʿ.
[26] The deacon Elishaʿ was among the signatories of the acts of the synod of Ishoʿyahb I in 585, on behalf of the bishop Habib of Pusang.
[27] The deacon Sargis was among the signatories of the acts of the synod of Ishoʿyahb I in 585, on behalf of the bishop Gabriel of Badisi and Qadistan.