On The Holy Trinity

We likewise believe in the Holy Ghost, the Lord and Giver of life, who proceeds from the Father and abides in the Son; who is adored and glorified together with the Father and the Son as consubstantial and co-eternal with Them; who is the true and authoritative Spirit of God and the source of wisdom and life and sanctification; who is God together with the Father and the Son and is so proclaimed; who is uncreated, complete, creative, almighty, all-working, all-powerful, infinite in power; who dominates all creation but is not dominated; who deifies but is not deified; who fills but is not filled; who is shared in but does not share; who sanctifies but is not sanctified; who, as receiving the intercessions of all, is the Intercessor; who is like the Father and the Son in all things; who proceeds from the Father and is communicated through the Son and is participated in by all creation; who through Himself creates and gives substance to all things and sanctifies and preserves them; who is distinctly subsistent and exists in His own Person indivisible and inseparable from the Father and the Son; who has all things whatsoever the Father and the Son have except the being unbegotten and the being begotten. For the Father is uncaused and unbegotten, because He is not from anything, but has His being from Himself and does not have from any other anything whatsover that He has. Rather, He Himself is the principle and cause by which all things naturally exist as they do. And the Son is begotten of the Father, while the Holy Ghost is Himself also of the Father—although not by begetting, but by procession. Now, we have learned that there is a difference between begetting and procession, but what the manner of this difference is we have not learned at all. However, the begetting of the Son and the procession of the Holy Ghost from the Father are simultaneous.

†St. John of Damascus
An Exact Exposition of the Orthodox Faith, Writings, The Fathers of the Church, pp 183-184