The writer sums up the argument he had commenced, and confirms the statement that unity is signified by the terms finger and right hand, from the fact that the works of God are the same as are the works of hands; and that those of hands are the same as those of fingers; and lastly, that the term hand applies equally to the Son and the Spirit, and that of finger applies to the Spirit and the Son.
29. But what wonder is it if He Who Himself needs no sanctification, but abounds therewith, sanctifies each man; since, as I said, we have been taught that His Majesty is so great, that the Holy Spirit seems to be as inseparable from God the Father as the finger is from the body?
30. But if any one thinks that this should be referred not to the oneness of power, but to its lessening, he indeed will fall into such madness as to appear to fashion the Father, Son, and Holy Spirit as it were into one bodily form, and to picture to himself certain distinctions of its members.
31. But let them learn, as I have often said, that not inequality but unity of power is signified by this testimony; inasmuch as things which are the works of God are also the works of hands, and we read that the same are the works of fingers. For it is written: “The heavens declare the glory of God, and the firmament shows the work of His hands;” and elsewhere: “In the beginning You founded the earth, O Lord; and the heavens are the works of Your hands.” So, then, the works of the hands are the same as the works of God. There is not therefore any distinction of the work according to the kind of bodily members, but a oneness of power.
32. But those which are the works of the hands are also the works of the fingers, for it is equally written: “For I will behold Your heavens, the works of Your fingers, the moon, and the stars, which You have established.” What less are the fingers here said to have made than the hands, since they made the same as the hands, as it is written: “For You, Lord, have made me glad through Your work, and in the works of Your hands will I rejoice.”
33. And yet since we read that the Son is the hand (for it is written: “Hath not My Hand made all these things?” and elsewhere: “I will place you in the cleft of the rock, and I will cover you with Mine hand, I have placed My hand under the covering of the rock,” which refers to the mystery of the Incarnation, because the eternal Power of God took on Itself the covering of a body), it is certainly clear that Scripture used the term hand both of the Son and of the Holy Spirit.
34. And again, since we read that the Spirit is the finger of God, we think that fingers [in the plural] are spoken of to signify the Son and Spirit. Lastly, that he may state that he received the sanctification both of the Son and of the Spirit, a certain saint says: “Your hands have made me and fashioned me.”
Source: On the Holy Spirit (New Advent)