St. Ambrose commences his argument by complimenting the Emperor, both for his faith and for the restitution of the Basilica to the Church; then having urged that his opponents, if they affirm that the Holy Spirit is not a servant, cannot deny Him to be above all, adds that the same Spirit, when He said, “All things serve You,” showed plainly that He was distinct from creatures; which point he also establishes by other evidence.
19. The Holy Spirit, then, is not among but above all things. For (since you, most merciful Emperor, are so fully instructed concerning the Son of God as to be able yourself to teach others) I will not detain you longer, as you desire and claim to be told something more exactly [concerning Him], especially since you lately showed yourself to be so pleased by an argument of this nature, as to command the Basilica to be restored to the Church without any one urging you.
20. So, then, we have received the grace of your faith and the reward of our own; for we cannot say otherwise than that it was of the grace of the Holy Spirit, that when all were unconscious of it, you suddenly restored the Basilica. This is the gift, I say, this the work of the Holy Spirit, Who indeed was at that time preached by us, but was working in you.
21. And I do not regret the losses of the previous time, since the sequestration of that Basilica resulted in the gain of a sort of usury. For you sequestrated the Basilica, that you might give proof of your faith. And so your piety fulfilled its intention, which had sequestered that it might give proof, and so gave proof as to restore. I did not lose the fruit, and I have your judgment, and it has been made clear to all that, with a certain diversity of action, there was in you no diversity of opinion. It was made clear, I say, to all, that it was not of yourself that you sequestrated, that it was of yourself when you restored it.
22. Now let us establish by evidence what we have said. The first point in the discussion is that all things serve. Now it is clear that all things serve, since it is written: “All things serve You.” This the Spirit said through the prophet. He did not say, We serve, but, “serve You,” that you might believe that He Himself is excepted from serving. So, then, since all things serve, and the Spirit does not serve, the Holy Spirit is certainly not included among all things.
23. For if we say that the Holy Spirit is included among all things, certainly when we read that the Spirit searches the deep things of God, we deny that God the Father is over all. For since the Spirit is of God, and is the Spirit of His mouth, how can we say that the Holy Spirit is included among all things, seeing that God, Whose is the Spirit, is over all, possessing certainly fullness of perfection and perfect power.
25. But lest the objectors should think that the Apostle was in error, let them learn whom he followed as his authority for his belief. The Lord said in the Gospel: “When the Paraclete has come, Whom I will send to you from My Father, even the Spirit of Truth which proceeds from the Father, He shall bear witness of Me.” So the Holy Spirit both proceeds from the Father, and bears witness of the Son. For the witness Who is both faithful and true bears witness of the Father, than which witness nothing is more full for the expression of the Divine Majesty, nothing more clear as to the Unity of the Divine Power, since the Spirit has the same knowledge as the Son, Who is the witness and inseparable sharer of the Father's secrets.
26. He excludes, then, the fellowship and number of creatures from the knowledge of God, but by not excluding the Holy Spirit, He shows that He is not of the fellowship of creatures. So that the passage which is read in the Gospel: “For no man has seen God at any time, save the Only-begotten Son Who is in the bosom of the Father He has declared Him,” also pertains to the exclusion of the Holy Spirit. For how has He not seen God Who searches even the deep things of God? How has He not seen God Who knows the things which are of God? How has He not seen God Who is of God? So, since it is laid down that no one has seen God at any time, whereas the Holy Spirit has seen Him, clearly the Holy Spirit is excepted. He, then, is above all Who is excluded from all.
Source: On the Holy Spirit (New Advent)