The Angelic Doctor Against the Errors of the Greeks - Chapter 13

 How by the name of Christ is to be understood the Holy Spirit.

Doubtful seems the passage of Cyril in his Thesaurus where he says that sometimes by the title Christ is understood the Holy Spirit. He states: “The Apostle by the name of Christ calls the Holy Spirit Christ; for he says: Christ is in you, although your bodies are dead, etc. (Rom. 8:10), and a little further on:”The Holy Spirit working in the name of Christ and representing Christ in himself is said to receive the name of Christ and to be called Christ by the Apostle.” 

To attribute, however, the name of one person to another seems incompatible with the distinction between persons. For, as the Father is never called the Son,  and vice versa, so the Son is never called the Holy Spirit, and vice versa. Hence, the term Christ may never be predicated of the Holy Spirit; nor can it stand for the Holy Spirit.

But it should be observed that this Father does not say the Spirit is called Christ or receives the name of Christ, as if Christ is predicated of the Holy Spirit or vice versa, for this would smack of impious Sabellianism; but the Holy Spirit is understood included in the term Christ by reason of concomitance, just as wherever the Father is present, there is the Son. Hence, the same Father interjects: “Did the preacher of truth”, namely the Apostle, “deny the distinction of persons, as Sabellius did? No. Rather he was at pains to point out to the Church that the Holy Spirit has the same nature as the Son.”