to mean that the Spirit is subordinated to the Son as the Son is subordinated to the Father. ‘The Spirit of the Son’ then means the Son’s mode of efficacy—a way in which the Son exercises his power—but not an independent divine person, over against the Son; just as ‘the Spirit of the Father’ means the Father’s mode of efficacy, but not an independent divine person, over against the Father. Tertullian’s famous trinitarian formula, una substantia—tres personae, asserts the divine personhood of the