The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 28

That to demonstrate the procession of the Holy Spirit the Greek and Latin Doctors use the same arguments.

That the Greek Doctors use the same arguments to demonstrate the procession of the Holy Spirit as do the Latin Doctors should also be pondered. In his book on the procession of the Holy Spirit  Anselm argues for the procession of the Holy Spirit on these grounds that Father and Son are of one essence. From this it follows that Father and Son do not differ from each other except that the former is the Father and the latter the Son. To have the Holy Spirit proceeding from himself pertains neither to the notion of paternity nor to that of filiation; for the Father is not called Father because the Holy Spirit proceeds from him. Nor is it contrary to the notion of filiation that the Son should have the Spirit proceeding from him. Hence, the only remaining conclusion is that to have the Holy Spirit proceeding from oneself is common to Father and Son.

And similarly Nicetas commenting on John argues thus: “From the fact that the Son has in his essence everything belonging to the Father, he also has the Spirit.”  And Cyril says in his Thesaurus: “The Apostle says that the Spirit of Christ and the Spirit of the Father are truly one and not many, because all things belonging to the Father truly and in the proper sense pass naturally to the true Son.” 

From this, however, it is clear that when in the Gospel the Holy Spirit is said to proceed from the Father, he is to be understood to proceed as well as from the Son, even though in the Gospel this is not added. For those things which are predicated of the Father must be understood of the Son as well,  even when predicated exclusively; as stated in John 17:3: That they may know you, the only God. And in 1 Tim. 6:15: Whom, that is, Christ, God will cause to appear in his own time- God, the blessed and only Ruler, the King of kings and Lord of lords, who alone is immortal. For these things are understood as true of the Son as well, because Father and Son are one in essence, as is asserted in John 10: 30: I and the Father are one.

Since, therefore, to have the Spirit proceeding from oneself is common to Father and Son, exactly as anything predicated of them essentially, as is clear from what has been said, then when it is said in the Gospel (cf. John 15: 26) that the Holy Spirit proceeds from the Father, he must be understood to proceed also from the Son; just as when in the same Creed the Father is said to be the Almighty, the Creator of things visible and invisible, the Son must be understood equally so.