Theodoret of Cyrus: Scripture has Innumerable Proofs of Trinitarian Orthodoxy

In the Holy Trinity, by contrast, we understand three substances, united without confusion, and subsisting of themselves. God the Word has been begotten of the Father before the ages but is inseparable from the one who begat him, and the most Holy Spirit proceeds from the God and Father, and is also understood as an individual substance: "The one and same Spirit activates everything and allots to each one individually as he wishes." There is no need, however, to go on at length; one can find innumerable proofs of this doctrine in holy Scripture.

- Theodoret of Cyrus (around A.D. 393 to around A.D. 457), Questions on the Octateuch, Question 20 on Genesis, pp. 55-57 (2007), Robert C. Hill translator.