And again, if the impious say, How will there not be three Gods and three Persons, on the supposition that they have one and the same divinity?— we shall reply: Just because God is the Cause and Father of the Son; and this Son is the image and offspring of the Father, and not His brother; and the Spirit in like manner is the Spirit of God, as it is written, “God is a Spirit.” And in earlier times we have this declaration from the prophet David: “By the word of the Lord were the heavens established, and all the power of them by the breath (spirit) of His mouth.” And in the beginning of the book of the creation it is written thus: “And the Spirit of God moved upon the face of the waters.” And Paul in his Epistle to the Romans says “But you are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you.” And again he says: “But if the Spirit of Him that raised up Jesus from the dead dwell in you, He that raised up Christ from the dead shall also quicken your mortal bodies by His Spirit that dwells in you.” And again: “As many as are led by the Spirit of God, they are the sons of God. For you have not received the spirit of bondage again to fear; but you have received the Spirit of adoption, whereby we cry, Abba, Father.” And again: “I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost.” And again: “Now the God of hope fill you with all joy and peace in believing, that you may abound in hope, by the power of the Holy Ghost.”
Source: A Sectional Confession of Faith (New Advent)