“Trinitarian Cooperation for our Salvation”: Ambrose of Milan's “De Spiritu Sancto”

While some have dismissed the trinitarian theology of Ambrose of Milan, he actually helped to shape subsequent theology in the West by employing the principle that "the same operations imply the same substance." That is, whenever the Father, Son, or Holy Spirit acts in Scripture, Ambrose believes one can find a place where another divine Person performs that same action, which in turn demonstrates their like substance. Ambrose's overarching purpose in his De Spiritu Sancto is to controvert his Homoian opponents' position that the various operations of the Father, Son, and Holy Spirit in salvation history indicate their subordinate positions to one another by stating that the Persons actually share similar roles and thus have the same substance, a principle he found in his Greek pneumatological sources—Didymus, Basil, and Athanasius—but which became far more central to his own trinitarian theology than to theirs.