That the Holy Spirit is in every conception separable from the Father and the Son, alike in the creation of perceptible objects, in the dispensation of human affairs, and in the judgment to come.
37. Let us then revert to the point raised from the outset, that in all things the Holy Spirit is inseparable and wholly incapable of being parted from the Father and the Son. St. Paul, in the passage about the gift of tongues, writes to the Corinthians, “If you all prophesy and there come in one that believes not, or one unlearned, he is convinced of all, he is judged of all; and thus are the secrets of the heart made manifest; and so falling down on his face he will worship God and report that God is in you of a truth.” If then God is known to be in the prophets by the prophesying that is acting according to the distribution of the gifts of the Spirit, let our adversaries consider what kind of place they will attribute to the Holy Spirit. Let them say whether it is more proper to rank Him with God or to thrust Him forth to the place of the creature. Peter's words to Sapphira, “How is it that you have agreed together to tempt the Spirit of the Lord? You have not lied unto men, but unto God,” show that sins against the Holy Spirit and against God are the same; and thus you might learn that in every operation the Spirit is closely conjoined with, and inseparable from, the Father and the Son. God works the differences of operations, and the Lord the diversities of administrations, but all the while the Holy Spirit is present too of His own will, dispensing distribution of the gifts according to each recipient's worth. For, it is said, “there are diversities of gifts, but the same Spirit; and differences of administrations, but the same Lord; and there are diversities of operations, but it is the same God which works all in all.” “But all these,” it is said, “works that one and the self-same Spirit, dividing to every man severally as He will.” It must not however be supposed because in this passage the apostle names in the first place the Spirit, in the second the Son, and in the third God the Father, that therefore their rank is reversed. The apostle has only started in accordance with our habits of thought; for when we receive gifts, the first that occurs to us is the distributer, next we think of the sender, and then we lift our thoughts to the fountain and cause of the boons.
Source: De Spiritu Sancto (New Advent)