Defining Christ describes the theological developments of the Church of the East in the Persian Empire in the seventh century, when Arab groupings were seizing power. This Church defined and emphasized the difference between the divine and human nature of Christ in such a way that it had already come in strong conflict with other Christian denominations. Now, it also had to deal with new rulers who recognized Christ as a unique human being, but rejected the belief in his divinity. The book investigates to what extent the leaders of this Church adapted their statements concerning Christ to the rapidly changing circumstances and views. Most attention is thereby given to the 106 letters of Isho'yahb III, written between about 625 and 659. These letters shed an interesting light on this period and for a better understanding they are set in the context of wider theological and political developments.