The same Unity may also be recognized from the fact that the Spirit is called Finger, and the Son Right Hand; for the understanding of divine things is assisted by the usage of human language. The tables of the law were written by this Finger, and they were afterwards broken, and the reason. Lastly, Christ wrote with the same Finger; yet we must not admit any inferiority in the Spirit from this bodily comparison.
11. So, too, the Spirit is also called the Finger of God, because there is an indivisible and inseparable communion between the Father, the Son, and the Holy Spirit. For as the Scripture called the Son of God the Right Hand of God, as it is said: “Your Right Hand, O Lord, is made glorious in power. Your Right Hand, O Lord, has dashed in pieces the enemy;” so the Holy Spirit is called the Finger of God, as the Lord Himself says: “But if I by the Finger of God cast out devils.” For in the same place in another book of the Gospel He named the Spirit of God, as you find: “But if I by the Spirit of God cast out devils.”
12. What, then, could have been said to signify more expressly the unity of the Godhead, or of Its working, which Unity is according to the Godhead of the Father, or of the Son, or of the Holy Spirit, than that we should understand that the fullness of the eternal Godhead would seem to be divided far more than this body of ours, if any one were to sever the unity of Substance, and multiply Its powers, whereas the eternity of the same Godhead is one?
13. For oftentimes it is convenient to estimate from our own words those things which are above us, and because we cannot see those things we draw inferences from those which we can see. “For the invisible things of Him,” says the Apostle, “from the creation of the world are clearly seen, being understood by those things which are made.” And he adds: “His eternal power also and Godhead.” Of which one thing seems to be said of the Son, and another of the Holy Spirit; that in the same manner as the Son is called the eternal Power of the Father, so, also, the Spirit, because He is divine, should be believed to be His eternal Godhead. For the Son, too, because He ever lives, is eternal life. This Finger, then, of God is both eternal and divine. For what is there belonging to God which is not eternal and divine?
14. With this Finger, as we read, God wrote on those tables of stone which Moses received. For God did not with a finger of flesh write the forms and portions of those letters which we read, but gave the law by His Spirit. And so the Apostle says: “For the Law is spiritual, which, indeed, is written not with ink, but with the Spirit of the living God; not in tables of stone, but on fleshy tables of the heart.” For if the letter of the Apostle is written in the Spirit, what hinders us from believing that the Law of God was written not with ink, but with the Spirit of God, which certainly does not stain but enlightens the secret places of our heart and mind?
14. Now it was written on tables of stone, because it was written in a type, but the tables were first broken and cast out of the hands of Moses, because the Jews fell away from the works of the prophet. And fitly were the tables broken, not the writing erased. And do you see that your table be not broken, that your mind and soul be not divided. Is Christ divided? He is not divided, but is one with the Father; and let no one separate you from Him. If your faith fails, the table of your heart is broken. The coherence of your soul is lessened if you do not believe the unity of Godhead in the Trinity. Your faith is written, and your sin is written, as Jeremiah said: “Your sin, O Judah, is written with a pen of iron and the point of a diamond. And it is written,” he says, “on your breast and on your heart.” The sin, therefore, is there where grace is, but the sin is written with a pen, grace is denoted by the Spirit.
15. With this Finger, also, the Lord Jesus, with bowed head, mystically wrote on the ground, when the adulteress was brought before Him by the Jews, signifying in a figure that, when we judge of the sins of another, we ought to remember our own.
16. And lest, again, because God wrote the Law by His Spirit, we should believe any inferiority, as it were, concerning the ministry of the Spirit, or from the consideration of our own body should think the Spirit to be a small part of God, the Apostle says, elsewhere, that he does not speak with words of human wisdom, but in words taught by the Spirit, and that he compares spiritual things with spiritual; but that the natural man receives not the things which pertain to the Spirit of God. For he knew that he who compared divine with carnal things was among natural things, and not to be reckoned among spiritual men; “for they are foolishness,” he says, “unto him.” And so, because he knew that these questions would arise among natural men, foreseeing the future he says: “For who has known the mind of the Lord, that he may instruct Him? But we have the mind of Christ.”
Source: On the Holy Spirit (New Advent)