John of Damascus served Umayyad caliphs while writing Christian theology in Greek.

Evidence

Yuhannâ b. Mansûr b. Sarjûn, better known as John of Damascus, bridged two worlds. Melkite presbyter and monk, John wrote extensively in Greek on a variety of theological issues. Yet he was also one of the chief financial administrators of the Umayyad caliphs ‘Abd al-Malik (685–705) and Walîd I (705–15). John’s family had long been prominent in Damascus; indeed, it was probably his grandfather, Mansûr b. Sarjûn, who surrendered the city to Khâlid b. al-Walîd in 635. John’s father, apparently, was an Arabic Christian who spoke no Greek; one Greek writer praises John for having learned Greek so fast. — John V. Tolan, Saracens, 338