The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 8

That the Holy Spirit is from the Father through the Son.

It is also taught by the aforesaid Doctors that the Holy Spirit is from the Father through the Son. For in his Thesaurus Cyril says: “God the Father by his own right hand, that is, by his power and wisdom, brought all things into being in his one true and life-giving and divinizing Spirit coessential with himself, whom in essence he spirated from himself through the same Son by nature coeternal with him.”  And Basil says: “The Son is not the Son of the Spirit because the Spirit is from God the Father through the Son.”  And writing against Eunomius the same Saint says: “If from your own word, O you who resist the truth, from your very own intelligence your produce a spirit by beating the air with a word not of the same kind, do you doubt the Holy Spirit proceeds in an intellectual manner from the Father through the only-begotten Son?” And further on: “The Son calls himself the Word of the Father and unhesitatingly declares to us that the Spirit is of the Father through himself, the Word.” 

Thus, it is shown conclusively that the Spirit is from the Son. For it was remarked above  that when anyone is said to work through something, that something must be a principle of operation for the agent, or at the least must be a principle of operation for that in which the operation terminates. But the Son cannot be a principle of spiration for the Father. Hence, if the Father spirates the Holy Spirit through the Son, it necessarily follows that the Son is a principle of the Holy Spirit.

The same is proven from the statement of Gregory of Nyssa: “We hold the Holy Spirit is from the Father through the mediation of the Son.”  For so the Holy Spirit is said to be from the Father through the Son, in as much as the Father is the principle of the Son, and the Son is the principle of the Holy Spirit.