Philoxenos of Mabbug (ca. 440s–523) wrote prolifically in defense of his theological positions. Especially important to him was the unity of Christ’s nature, a position he thought that the most powerful Christians of his day had mistakenly abandoned. Written toward the end of his life and after his removal from his office as bishop in favor of a supporter of the Chalcedonian Definition, this letter outlines the fine details of Philoxenos’s miaphysite Christology. For him, the process by which the Word had become human was key for understanding the entirety of Christianity; the questions about how and when this “humanification,” as he called it, had happened are pursued in the letter to an extent that can seem excessive, but the mechanics of humanification mattered to Philoxenos for two reasons. First, a mistaken idea about this process could lead one to think wrongly about the status of Christ, that of his human mother Mary, and about the efficacy of salvation itself. Second, getting the details of humanification wrong could lead people to think that, in addition to the Trinity of Father, Son, and Holy Spirit, there was a fourth deity, named the Lord or the Son, who resides outside the Trinity.