10 This examination of the passages before us is, so far as my ability goes, sufficient. Now let us turn the discussion on those who attack the Holy Spirit, and cast down every high thing of their intellect that exalts itself against the knowledge of God. You say that the Holy Ghost is a creature. And every creature is a servant of the Creator, for “all are your servants.” If then He is a servant, His holiness is acquired; and everything of which the holiness is acquired is receptive of evil; but the Holy Ghost being holy in essence is called “fount of holiness.” Therefore the Holy Ghost is not a creature. If He is not a creature, He is of one essence and substance with the Father. How, tell me, can you give the name of servant to Him Who through your baptism frees you from your servitude? “The law,” it is said, “of the Spirit of life has made me free from the law of sin.” But you will never venture to call His nature even variable, so long as you have regard to the nature of the opposing power of the enemy, which, like lightning, is fallen from heaven and fell out of the true life because its holiness was acquired, and its evil counsels were followed by its change. So when it had fallen away from the Unity and had cast from it its angelic dignity, it was named after its character “Devil,” its former and blessed condition being extinct and this hostile power being kindled.
Furthermore if he calls the Holy Ghost a creature he describes His nature as limited. How then can the two following passages stand? “The Spirit of the Lord fills the world;” and “Whither shall I go from your Spirit?” But he does not, it would seem, confess Him to be simple in nature; for he describes Him as one in number. And, as I have already said, everything that is one in number is not simple. And if the Holy Spirit is not simple, He consists of essence and sanctification, and is therefore composite. But who is mad enough to describe the Holy Spirit as composite, and not simple, and consubstantial with the Father and the Son?
Source: Letters (New Advent)