This article analyses the reception of the story of Constantine in Iraqi Christian circles in the ninth and tenth centuries. It situates the use of the story against the broader historiographic context in which the history of the Roman church was imported wholesale into Iraq in the sixth century to buttress its identity as an orthodox church. It argues that the legacy of Eusebius was respected but not followed in its details. Instead, the memory of Constantine and his family was dominated by the Doctrina Addai and the Julian Romance, pseudo-histories composed in Syriac in Edessa in the fifth and sixth centuries. Within an Islamicate environment, Constantine was remembered chiefly for his role in establishing a Nicene orthodoxy, which was shared by all major Christian confessions in the caliphate, and for his role in the cult of the True Cross, a strong symbol that continued to divide Christians and Muslims.