This paper is about Yaḥyā Ibn ‘Adī, a 10th century CE Christian philosopher and theologian, and his views on divine oneness (tawḥīd). Yaḥyā was a contemporary and peer of the foundational Islamic philosopher al-Fārābī (d. 950 CE), having studied alongside him under the tutelage of the scholarch of the Baghdad Peripatetics, Abu Bishr Mattā ibn Yūnus (d. 940 CE). Yaḥyā lived during the peak of the Graeco-Arabic translation movement during which Jews, Muslims, and Christians worked alongside one another with Abbasid state support to translate and study a wide range of Greek scientific, medical, and philosophical texts. In his Maqāla fī al-Tawḥīd (Treatise on Divine Oneness), Yaḥyā presents and refutes four ways that something may be called ‘one,’ before offering his own positive account of oneness as it applies to God, who for the Christian Yaḥyā is unique from one perspective and multiple from another. We focus on Yaḥya’s presentation and refutation of the second sense in which something which ‘has no equal’ (lā naẓīr lahu) is one. We argue that Yaḥyā here likely has al-Fārābī in mind, who describes in his Mabādi’ ahl al-Madīna al-Fāḍila (The Virtuous City) the absolute unity of the First Existent (al-mawjūd al-awwal). His existence has no equal; it shares nothing in common at all with the existence of any being. Examining Yaḥyā’s demonstrative proofs against the incoherence of such an understanding of divine oneness, we will evaluate Yaḥyā’s proofs and argue that they are sound and effective. They represent an early rationalist critique of al-Fārābī’s tendencies towards a theologia negativa. They also bear witness to the productive intellectual diversity of 10th century Abbasid Baghdad, showing that ‘Islamic’ philosophy includes important contributions by non-Muslims.