VI. And this equality which the Son has with the Father, the Holy Ghost also has
Accordingly, dearly-beloved, let us despise the vain and blind cunning of ungodly heretics, which flatters itself over its crooked interpretation of this sentence, and when the Lord says, “All things that the Father has are Mine,” does not understand that it takes away from the Father whatever it dares to deny to the Son, and is so foolish in matters even which are human as to think, that what is His Father's has ceased to belong to His Only-begotten, because He has taken on Him what is ours.
Mercy in the case of God does not lessen power, nor is the reconciliation of the creature whom He loves a falling off of Eternal glory. What the Father has the Son also has, and what the Father and the Son have, the Holy Ghost also has, because the whole Trinity together is One God. But this Faith is not the discovery of earthly wisdom nor the conviction of man's opinion: the Only-begotten Son has taught it Himself, and the Holy Ghost has established it Himself, concerning Whom no other conception must be formed than is formed concerning the Father and the Son.
Because albeit He is not the Father nor the Son, yet He is not separable from the Father and the Son: and as He has His own personality in the Trinity, so has He One substance in Godhead with the Father and the Son, filling all things, containing all things, and with the Father and the Son controlling all things, to Whom is the honour and glory for ever and ever. Amen.
Source: Sermons (New Advent)