To those who contend that the Spirit because He is called the Finger is less than the Father, St. Ambrose replies that this would also tend to the lessening of the Son, Who is called the Right Hand. That these names are to be referred only to the Unity, for which reason Moses proclaimed that the whole Trinity worked in the passage of the Red Sea. And, indeed, it is no wonder that the operation of the Spirit found place there, where there was a figure of baptism, since the Scripture teaches that the Three Persons equally sanctify and are operative in that sacrament.
17. But if any one is still entangled in carnal doubts, and hesitates because of bodily figures, let him consider that he cannot think rightly of the Son who can think wrongly of the Spirit. For if some think that the Spirit is a certain small portion of God, because He is called the Finger of God, the same persons must certainly maintain that a small portion only is in the Son of God, because He is called the Right Hand of God.
18. But the Son is called both the Right Hand and the Power of God; if, then, we consider our words, there can be no perfection without power; let them therefore take care lest they think that which it is impious to say, namely, that the Father being but half perfect in His own Substance received perfection through the Son, and let them cease to deny that the Son is co-eternal with the Father. For when did the Power of God not exist? But if they think that at any time the Power of God existed not, they will say that at some time Perfection existed not in God the Father, to Whom they think that Power was at some time wanting.
19. But, as I said, these things are written that we may refer them to the Unity of the Godhead, and believe that which the Apostle said, that the fullness of the Godhead dwells bodily in Christ, which dwells also in the Father, and dwells in the Holy Spirit; and that, as there is a unity of the Godhead, so also is there a unity of operation.
20. And this may also be gathered from the Song of Moses, for he, after leading the people of the Jews through the sea, acknowledged the operation of the Father, the Son, and the Holy Spirit, saying: “Your Right Hand, O Lord, is glorious in power, Your Right Hand, O Lord, has dashed in pieces the enemy.” Here you have his confession of the Son and of the Father, Whose Right Hand He is. And farther on, not to pass by the Holy Spirit, He added: “You sent Your Spirit and the sea covered them, and the water was divided by the Spirit of Your anger.” By which is signified the unity of the Godhead, not an inequality of the Trinity.
21. You see, then, that the Holy Spirit also co-operated with the Father and the Son, so that just as if the waves were congealed in the midst of the sea, a wall as it were of water rose up for the passage of the Jews, and then, poured back again by the Spirit, overwhelmed the people of the Egyptians. And many think that from the same origin the pillar of cloud went before the people of the Jews by day, and the pillar of fire by night, that the grace of the Spirit might protect His people.
22. Now that this operation of God, which the whole world rightly wonders at, did not take place without the work of the Holy Spirit, the Apostle also declared when he said that the truth of a spiritual mystery was prefigured in it, for we read as follows: “For our fathers were all under the cloud, and all passed through the sea, and were all baptized in Moses in the cloud and in the sea, and did all eat the same spiritual meat, and did all drink the same spiritual drink.”
23. For how without the operation of the Holy Spirit could there be the type of a sacrament, the whole truth of which is in the Spirit? As the Apostle also set forth, saying: “But you were washed, but you were sanctified, but you were justified in the Name of our Lord Jesus Christ, and in the Spirit of our God.”
24. You see, then, that the Father works in the Son, and that the Son works in the Spirit. And therefore do not doubt that, according to the order of Scripture, there was in the figure that which the Truth Himself declared to be in the truth. For who can deny His operation in the Font, in which we feel His operation and grace?
25. For as the Father sanctifies, so, too, the Son sanctifies, and the Holy Spirit sanctifies. The Father sanctifies according to that which is written: “The God of peace sanctify you, and may your spirit, soul, and body be preserved entire without blame in the day of our Lord Jesus Christ.” And elsewhere the Son says: “Father, sanctify them in the truth.”
26. But of the Son the same Apostle said: “Who was made unto us wisdom from God, and righteousness, and sanctification, and redemption.” Do you see that He was made sanctification? But He was made so unto us, not that He should change that which He was, but that He might sanctify us in the flesh.
27. And the Apostle also teaches that the Holy Spirit sanctifies. For he speaks thus: “We are bound to give thanks to God always for you, brethren dearly beloved of the Lord; because God chose you as first-fruits unto salvation, in sanctification of the Spirit, and belief of the truth.”
28. So, then, the Father sanctifies, the Son also sanctifies, and the Holy Spirit sanctifies; but the sanctification is one, for baptism is one, and the grace of the sacrament is one.
Source: On the Holy Spirit (New Advent)