The argument of this chapter is that the instrument doctrine, if held, sharpens our understanding of other mysteries of the Christian faith, including the hypostatic union, the Church, and the sacrament of the Eucharist. One reason that Aquinas held to the doctrine was the clarity it brought to these other mysteries. With Christology, the benefits are grammatical and speculative. Calling Christ’s humanity an instrument leads us to think of the one who possesses it, namely, the Logos, and so it can render single-subject Christology clearer to the intellect. If Christ’s humanity is held to be an instrument, it also implicates in our understanding the divine power through which the Logos works. With the theology of the Church, the instrument doctrine supports the biblical idea that Christ sends the Holy Spirit on the Church as a human being, and not only as God. And with the theology of the Eucharist, the instrument doctrine assists Catholic faith in perceiving how Jesus gives us a share of the divine life through sacramental communion.