Despite instances of persecution against Christians by Muslim rulers in the early centuries of Islamic rule, there is a great deal of evidence for tolerance as the official policy of Muslim rulers with respect to Christians. In the present article, Alphonso Mingana presents the Arabic text and English translation of a letter that provides precisely this kind of evidence. The letter was written in A.D. 1138 by the Caliph of Baghdad, Muktafi II, and it was sent to Patriarch Abdisho III. The letter, beyond mere tolerance, actually places the authority of the Muslim state behind the leaders of the local church—in this case, the leaders of the Assyrian Church of the East—including the use of Muslim courts to adjudicate cases involving conflicts among Christian groups.