Covenant of Umar
For many people, any discussion of Christianity in the Islamic period must involve the text, or rather family of texts, called the Covanant of Umar. Purportedly offered to the early caliph Umar b. al-Khattab by Christians at the time of the initial Islamic conquests, it became a later cornerstone of Islamic jurisprudence on the treatment of non-Muslims under Islamic rule, especially Jews and Christians. However, it is not nearly as obviously relevant to the history of Gulf Christianity as some might think.
For most of a century now, scholars have often dated the covenant, which exists in several forms, to the early eighth-century Umayyad caliph Umar b. Abd al-Aziz, who sought to place Umayyad government on a more overtly Islamic footing and issued a number of regulations for non-Muslims. The recent indispensable study of the matter, however, is Milka Levy-Rubin's Non-Muslims in the Early Islamic Empire. Through meticulous research, Levy-Rubin demonstrates that as late as about 800, the Covenant of Umar was one of several competing theo-juristic models for how Muslims should be regulating the non-Muslims among them.
Levy-Rubin argues more comprehensively than I have generally seen that most of the stipulations in the Covenant go back to Byzantine and Sasanian precedents, and one of her contributions is linking many of its stipulations to Sasanian social codes which used dress to distinguish within its strict class hierarchy. As Muslim society gradually took on some of the Sasanian ethos, religion replaced social class as the primary marker of social standing in the Islamic jurisprudential tradition.
A key moment in the imposition of the Covenant of Umar's stipulations was the reign of the Abbasid caliph al-Mutawakkil from 847-861. Others have highlighted how his reign marked a turn in which the caliphs moved away from trying to impose their own religious authority in favor of ruling as the agent of an independent class of ulama. A key development was ending the mihna, an inquisition by which the caliphs sought to impose the doctrine that the Qur'an was created and not eternal. It is perhaps relevant to this environment that al-Mutawakkil issued a decree implementing stipulations of the Covenant of Umar, which would subsequently become the standard reference as described above.
I began this post by linking it to my work on Gulf Christianity. The connection, or rather lack of connection, is that Gulf Christianity seems to have disappeared at about the same time the Covenant of Umar was being imposed. These two developments also do not appear to be related, since the Abbasid caliphs did not have control of the relevant areas of the Gulf at the time and, as I will eventually explain, there are other factors to explain Christianity's decline there.