The introduction states the biblical premise of the book’s argument. In Scripture, God saves human beings through the actions and sufferings of Christ in the flesh. St. Thomas Aquinas developed a theological account of the Incarnation that attempts to account for the way Scripture speaks, namely, that Christ’s humanity is the instrumental cause of salvation, or as the book calls it, "the instrument doctrine." The introduction then gives an overview of the book’s argument: this doctrine best accounts for how Jesus Christ saves Christians in virtue of his humanity. It outlines the argument of the seven following chapters.