20 For the present I forbear to expose their licence of speculation, some of them holding that the Paraclete Spirit comes from the Father or from the Son. For our Lord has not left this in uncertainty, for after these same words He spoke thus— I have yet many things to say unto you, but you cannot bear them now. When He, the Spirit of truth, has come, He shall guide you into all truth: for He shall not speak from Himself: but what things soever He shall hear, these shall He speak; and He shall declare unto you the things that are to come.
He shall glorify Me: for He shall receive of Mine and shall declare it unto you. All things whatsoever the Father has are Mine: therefore said I, He shall receive of Mine and shall declare it unto you</em>. Accordingly He receives from the Son, Who is both sent by Him, and proceeds from the Father. Now I ask whether to receive from the Son is the same thing as to proceed from the Father. But if one believes that there is a difference between receiving from the Son and proceeding from the Father, surely to receive from the Son and to receive from the Father will be regarded as one and the same thing.
For our Lord Himself says, Because He shall receive of Mine and shall declare it unto you. All things whatsoever the Father has are Mine: therefore said I, He shall receive of Mine and shall declare it unto you. That which He will receive—whether it will be power, or excellence, or teaching—the Son has said must be received from Him, and again He indicates that this same thing must be received from the Father. For when He says that all things whatsoever the Father has are His, and that for this cause He declared that it must be received from His own, He teaches also that what is received from the Father is yet received from Himself, because all things that the Father has are His.
Such a unity admits no difference, nor does it make any difference from whom that is received, which given by the Father is described as given by the Son. Is a mere unity of will brought forward here also? All things which the Father has are the Son's, and all things which the Son has are the Father's. For He Himself says, And all Mine are Yours, and Yours are Mine. It is not yet the place to show why He spoke thus, For He shall receive of Mine: for this points to some subsequent time, when it is revealed that He shall receive.
Now at any rate He says that He will receive of Himself, because all things that the Father had were His. Dissever if you can the unity of the nature, and introduce some necessary unlikeness through which the Son may not exist in unity of nature. For the Spirit of truth proceeds from the Father and is sent from the Father by the Son. All things that the Father has are the Son's; and for this cause whatever He Who is to be sent shall receive, He shall receive from the Son, because all things that the Father has are the Son's.
The nature in all respects maintains its law, and because Both are One that same Godhead is signified as existing in Both through generation and nativity; since the Son affirms that that which the Spirit of truth shall receive from the Father is to be given by Himself. So the frowardness of heretics must not be allowed an unchecked licence of impious beliefs, in refusing to acknowledge that this saying of the Lord,— that because all things which the Father has are His, therefore the Spirit of truth shall receive of Him—is to be referred to unity of nature.
Source: On the Trinity (New Advent)