From the fact that St. Paul has shown that the light of the Godhead which the three apostles worshipped in Christ is in the Trinity, it is made clear that the Spirit also is to be worshipped. It is shown from the words themselves that the Spirit is intended by the apostles. The Godhead of the same Spirit is proved from the fact that He has a temple wherein He dwells not as a priest, but as God: and is worshipped with the Father and the Son; whence is understood the oneness of nature in Them.
86. But does any one deny that the Godhead of the eternal Trinity is to be worshipped? Whereas the Scriptures also express the inexplicable Majesty of the Divine Trinity, as the Apostle says elsewhere: “Since God, Who said that light should shine out of darkness, shined in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ.”
87. The apostles truly saw this glory, when the Lord Jesus on the mount shone with the light of His Godhead: “The apostles,” it says, “saw it and fell on their face.” Do not you think that they even, as they fell, worshipped, when they could not with their bodily eyes endure the brightness of the divine splendour, and the glory of eternal light dulled the keenness of mortal sight? Or what else did they who saw His glory say at that time, except, “O come let us worship and fall down before Him”? For “God shined in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ.”
88. Who is He, then, Who shined that we might know God in the face of Jesus Christ? For he said, “God shined,” that the glory of God might be known in the face of Jesus Christ. Whom else do we think but the manifested Spirit? Or who else is there besides the Holy Spirit to Whom the power of the Godhead may be referred? For they who exclude the Spirit must necessarily bring in another, who may with the Father and the Son receive the glory of the Godhead.
89. Let us then go back to the same words: “It is God Who shined in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ.” You have Christ plainly set forth. For Whose glory is said to give light but that of the Spirit? So, then, he set forth God Himself, since he spoke of the glory of God; if of the Father, it remains that “He who said that light should shine out of darkness, and shine in our hearts,” be understood to be the Holy Spirit, for we cannot venerate any other with the Father and the Son. If, then, you understand the Spirit, Him also has the Apostle called God; it is necessary, then, that you also confess the Godhead of the Spirit, who now deny it.
90. But how shamelessly do you deny this, since you have read that the Holy Spirit has a temple. For it is written: “You are the temple of God, and the Holy Spirit dwells in you.” Now God has a temple, a creature has no true temple. But the Spirit, Who dwells in us, has a temple. For it is written: “Your members are temples of the Holy Spirit.”
91. But He does not dwell in the temple as a priest, nor as a minister, but as God, since the Lord Jesus Himself said: “I will dwell in them, and will walk among them, and will be their God, and they shall be My people.” And David says: “The Lord is in His holy temple.” Therefore the Spirit dwells in His holy temple, as the Father dwells and as the Son dwells, Who says: “I and the Father will come, and will make Our abode with him.”
92. But the Father abides in us through the Spirit, Whom He has given us. How, then, can different natures abide together? Certainly it is impossible. But the Spirit abides with the Father and the Son. Whence, too, the Apostle joined the Communion of the Holy Spirit with the grace of Jesus Christ and the love of God, saying: “The grace of our Lord Jesus Christ, and the love of God, and the Communion of the Holy Spirit be with you all.”
91. We observe, then, that the Father, the Son, and the Holy Spirit abide in one and the same [subject] through the oneness of the same nature. Therefore, He Who dwells in the temple has divine power, for as of the Father and of the Son, so are we also the temple of the Holy Spirit; not many temples, but one temple, for it is the temple of one Power.
Source: On the Holy Spirit (New Advent)