This is the first English translation of the two apologetic works by the 9th-century East Syrian theologian ʿAmmār al-Baṣrī. The Book of the Proof and The Book of Questions and Answers were written to defend Christian beliefs in the face of Muslim criticism. ʿAmmār's defense of the authenticity of the gospels, the Trinity, the divinity of Christ, the Incarnation, the death of Christ by crucifixion, the resurrection of Christ, and the nature of the afterlife, formed the first systematic theology in Arabic written in dialogue with Muslim thinkers of his time. This English translation should enable global readers to appreciate the depth of engagement of this virtually unknown theologian with his Islamic context, and to provide them with assistance in relating to the concerns that Muslims have today with Christian beliefs and practices.