This presentation examines the first generation of Christian theologians writing in Arabic, focusing on their awareness of developments in early Islamic thought surrounding two key concepts in theology and fiqh: raʾy (opinion) and bidʿa (innovation). Current research in Islamic studies generally examines the evolution of these categories based solely on intra-Islamic data, but Christian theologians provide extra-Islamic witnesses to how Muslim discourse around these terms affected Christian responses to Muslims' polemics against Christians. The West Syrian Abū Rāʾiṭa al-Takrītī (d. ca. 220/835) offers the clearest example of an apologetic argument denying that Christians innovate or construct their doctrine or practice based on opinion, but the paper also examines Theodore Abū Qurra (d. ca. 214/829) and the East Syrian ʿAmmār al-Baṣrī (d. ca. 225-235/840-850) as complementary witnesses to how Christians responded to this Islamic challenge.