Scripture teaches that God saves humanity through God's own actions and sufferings in Christ, thereby raising a key theological question: How can God use his own human actions and sufferings to bring about those things that he causes through divine power? To answer that question, J. David Moser here explores St. Thomas Aquinas's teaching that Christ's humanity is an instrument of the divinity. Offering an informed account of how Christian salvation happens through the Incarnation of Christ, he also poses a new set of questions about the Incarnation that Aquinas himself did not consider. In response to these questions, and in conversation with a wide range of theologians, including John Duns Scotus and Matthias Joseph Scheeben, Moser argues that the instrument doctrine, an underexplored and underappreciated idea, deepens our understanding of salvation that comes through the Incarnation of Jesus Christ. He also defends the instrument doctrine as a dogmatic theological topic worthy of consideration today.