In 1180, at the end of Manuel I Comnenus’s reign (1143-1180), a theologic controversy opposed the emperor to Patriarch Theodosius Boradiotes and to other members of the church, among whom Eustathius of Thessalonica. The dispute arose because of the emperor’s effort to modify the formula of abjuration for the converts from Islam to Christianity, eliminating the anathema against ‘the God of Mohammad’. The paper examines the reasons for, the development and the meaning of the controversy, comparing for the first time all the available sources: first and foremost Nicetas Choniates’s version in his Chronikè diegesis (book VIII) and in his Panoplia dogmatica (book XXVI), and then the synodal tomos of April 1180. The reasons for the controversy are viewed on one hand within the frame of XII century Turkish-Byzantine relationships (when several Turks converted to Christianity), on the other within the frame of the polemical Byzantine tradition against Islam. Manuel I Comnenus’s position can be interpreted as a minority tendency, distancing itself from the Byzantine polemical tradition against Islam and acknowledging the common monotheism of Christians and Muslims, who adore the same God: this view would deserve a greater emphasis in the historical reconstruction of ‘interreligious’ relationships between Islam and Christianity.