Chapter 5 — Part I: The Creed — Article III: Who Was Conceived by the Holy Ghost, Born of the Virgin Mary
Section 6. It is a principle of Christian faith that whatever God does outside Himself in creation is common to the Three Persons, and that one neither does more than, nor acts without another. But that one emanates from another, this only cannot be common to all; for the Son is begotten of the Father only, and the Holy Ghost proceeds from the Father and the Son. Anything, however, which proceeds from them extrinsically is the work of the Three Persons without difference of any sort, and of this latter description is the Incarnation of the Son of God.