John of Damascus
God is One, that is to say, one essence; and that He is known, and has His being in three subsistences, in Father, I say, and Son and Holy Spirit; and that the Father and the Son and the Holy Spirit are one in all respects.
Source details
A.D. 743 · Exposition of the Orthodox Faith, Book I, ch. 8
Checked word-for-word against Nicene and Post-Nicene Fathers, Series 2, Vol. 9 · 2026-07-17