ʿAmmār al-Baṣrī (d.c. 850) was the first Christian to write a systematic theology in Arabic, the language of the Muslim rulers of ʿAmmār’s Middle East. This study of his two works that were only discovered in the 1970’s seeks to analyse the way he defends Christian beliefs from criticism by Muslims over 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. ʿAmmār al-Baṣrī wrote his theology in dialogue with Muslim thinkers of his time and his work offers guidance to Christians in today’s world who live in Islamic contexts in how to relate Christian convictions to a Muslim audience.