17. Above, you read that “the Lord blessed him, and the Spirit of the Lord began to go with him.” Farther on it is said: “And the Spirit of the Lord came upon him.” Again he says: “If I be shaven, my strength will depart from me.” After he was shaven, see what the Scripture says: “The Lord,” he says, “departed from him.”
18. You see, then, that He Who went with him, Himself departed from him. The Same is, then, the Lord, Who is the Spirit of the Lord, that is, he called the Spirit of God, Lord, as also the Apostle says: “The Lord is the Spirit, now where the Spirit of the Lord is, there is liberty.” You find, then, the Holy Spirit called the Lord; for the Holy Spirit and the Son are not one Person [unus] but one Substance [unum].
19. In this place he used the word Power, and implied the Spirit. For as the Father is Power, so, too, the Son is Power, and the Holy Spirit is Power. Of the Son you have read that Christ is “the Power of God and the Wisdom of God.” We read, too, that the Father is Power, as it is written: “Ye shall see the Son of Man sitting at the right hand of the Power of God.” He certainly named the Father Power, at Whose right hand the Son sits, as you read: “The Lord said unto My Lord, Sit Thou on My right hand.” And the Lord Himself named the Holy Spirit Power, when He said: “Ye shall receive Power when the Holy Spirit cometh upon you.”
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 10 (Schaff and Wace, 1896), public domain (Public domain).