Chapter 15 — Of the Holy Spirit, that he is in God

The authority of Sacred Scripture not only reveals the existence in God of Father and Son, but also numbers the Holy Spirit with them. Thus our Lord says: Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit (Matt 28:19); and John declares: There are three who give testimony in heaven: the Father, the Word, and the Holy Spirit; and these three are one (1 John 5:7). Moreover, Sacred Scripture witnesses to a kind of procession of this same Holy Spirit, for it says: When the Counselor comes, whom I shall send to you from the Father, even the Spirit of truth, who proceeds from the Father, he will bear witness to me (John 15:26).