One of those sobering moments is when you realize that what you think about the Trinity has been expressed better than you can a mere five hundred years ago -- and a that time Calvin was quoting shurch fathers from a few centuries before his time. We cannot understand the mind of God: we are not the creator, the created. But we can comprehend the descriptions we have been given.
This should be a cause for worship, not speculation.
14. In asserting the divinity of the Spirit, the proof must be derived from the same sources. And it is by no means an obscure testimony which Moses bears in the history of the creation, when he says that the Spirit of God was expanded over the abyss or shapeless matter; for it shows not only that the beauty which the world displays is maintained by the invigorating power of the Spirit, but that even before this beauty existed the Spirit was at work cherishing the confused mass. Again, no cavils can explain away the force of what Isaiah says, "And now the Lord God, and his Spirit, has sent me," (Isa. 48:16), thus ascribing a share in the sovereign power of sending the prophets to the Holy Spirit. (Calvin in Acts 20:28). In this his divine majesty is clear.
But, as I observed, the best proof to us is our familiar experience. For nothing can be more alien from a creature, than the office which the Scriptures ascribe to him, and which the pious actually feel him discharging,--his being diffused over all space, sustaining, invigorating, and quickening all things, both in heaven and on the earth. The mere fact of his not being circumscribed by any limits raises him above the rank of creatures, while his transfusing vigour into all things, breathing into them being, life, and motion, is plainly divine. Again, if regeneration to incorruptible life is higher, and much more excellent than any present quickening, what must be thought of him by whose energy it is produced? Now, many passages of Scripture show that he is the author of regeneration, not by a borrowed, but by an intrinsic energy; and not only so, but that he is also the author of future immortality. In short, all the peculiar attributes of the Godhead are ascribed to him in the same way as to the Son. He searches the deep things of Gods and has no counsellor among the creatures; he bestows wisdom and the faculty of speech, though God declares to Moses (Exod. 4:11) that this is his own peculiar province. In like manner, by means of him we become partakers of the divine nature, so as in a manner to feel his quickening energy within us. Our justification is his work; from him is power, sanctification, truth, grace, and every good thought, since it is from the Spirit alone that all good gifts proceed. Particular attention is due to Paul's expression, that though there are diversities of gifts, "all these worketh that one and the self-same Spirit," (1 Cor. 12:11), he being not only the beginning or origin, but also the author; as is even more clearly expressed immediately after in these words "dividing to every man severally as he will." For were he not something subsisting in God, will and arbitrary disposal would never be ascribed to him. Most clearly, therefore does Paul ascribe divine power to the Spirit, and demonstrate that he dwells hypostatically in God.
15. Nor does the Scripture, in speaking of him, withhold the name of God. Paul infers that we are the temple of God, from the fact that "the Spirit of God dwelleth in us," (1 Cor. 3:16; 6:19; and 2 Cor. 6:16). Now it ought not to be slightly overlooked, that all the promises which God makes of choosing us to himself as a temple, receive their only fulfilment by his Spirit dwelling in us. Surely, as it is admirably expressed by Augustine (Ad Maximinum, Ep. 66), "were we ordered to make a temple of wood and stone to the Spirit, inasmuch as such worship is due to God alone, it would be a clear proof of the Spirit's divinity; how much clearer a proof in that we are not to make a temple to him, but to be ourselves that temple." And the Apostle says at one time that we are the temple of God, and at another time, in the same sense, that we are the temple of the Holy Spirit. Peter, when he rebuked Ananias for having lied to the Holy Spirit, said, that he had not lied unto men, but unto God. And when Isaiah had introduced the Lord of Hosts as speaking, Paul says, it was the Holy Spirit that spoke (Acts 28:25, 26). Nay, words uniformly said by the prophets to have been spoken by the Lord of Hosts, are by Christ and his apostles ascribed to the Holy Spirit. Hence it follows that the Spirit is the true Jehovah who dictated the prophecies. Again, when God complains that he was provoked to anger by the stubbornness of the people, in place of Him, Isaiah says that his Holy Spirit was grieved (Isa. 63:10). Lastly, while blasphemy against the Spirit is not forgiven, either in the present life or that which is to come, whereas he who has blasphemed against the Son may obtain pardon, that majesty must certainly be divine which it is an inexpiable crime to offend or impair. I designedly omit several passages which the ancient fathers adduced. They thought it plausible to quote from David, "By the word of the Lord were the heavens made, and all the host of them by the breath (Spirit) of his mouth," (Ps. 33:6), in order to prove that the world was not less the work of the Holy Spirit than of the Son. But seeing it is usual in the Psalms to repeat the same thing twice, and in Isaiah the spirit (breath) of the mouth is equivalent to word, that proof was weak; and, accordingly, my wish has been to advert briefly to those proofs on which pious minds may securely rest.
16. But as God has manifested himself more clearly by the advent of Christ, so he has made himself more familiarly known in three persons. Of many proofs let this one suffice. Paul connects together these three, God, Faith, and Baptism, and reasons from the one to the other--viz. because there is one faith he infers that there is one God; and because there is one baptism he infers that there is one faith. Therefore, if by baptism we are initiated into the faith and worship of one God, we must of necessity believe that he into whose name we are baptised is the true God. And there cannot be a doubt that our Saviour wished to testify, by a solemn rehearsal, that the perfect light of faith is now exhibited, when he said, "Go and teach all nations, baptising them in the name of the Father, and of the Son, and of the Holy Spirit," (Mt. 28:19), since this is the same thing as to be baptised into the name of the one God, who has been fully manifested in the Father, the Son, and the Spirit. Hence it plainly appears, that the three persons, in whom alone God is known, subsist in the Divine essence. And since faith certainly ought not to look hither and thither, or run up and down after various objects, but to look, refer, and cleave to God alone, it is obvious that were there various kinds of faith, there behaved also to be various gods. Then, as the baptism of faith is a sacrament, its unity assures us of the unity of God. Hence also it is proved that it is lawful only to be baptised into one God, because we make a profession of faith in him in whose name we are baptised. What, then, is our Saviour's meaning in commanding baptism to be administered in the name of the Father, and the Son, and the Holy Spirit, if it be not that we are to believe with one faith in the name of the Father, and the Son, and the Holy Spirit? But is this any thing else than to declare that the Father, Son, and Spirit, are one God? Wherefore, since it must be held certain that there is one God, not more than one, we conclude that the Word and Spirit are of the very essence of God. Nothing could be more stupid than the trifling of the Arians, who, while acknowledging the divinity of the Son, denied his divine essence. Equally extravagant were the ravings of the Macedonians, who insisted that by the Spirit were only meant the gifts of grace poured out upon men. For as wisdom understanding, prudence, fortitude, and the fear of the Lord, proceed from the Spirit, so he is the one Spirit of wisdom, prudence, fortitude, and piety. He is not divided according to the distribution of his gifts, but, as the Apostle assures us (1 Cor. 12:11), however they be divided, he remains one and the same.
17. On the other hand, the Scriptures demonstrate that there is some distinction between the Father and the Word, the Word and the Spirit; but the magnitude of the mystery reminds us of the great reverence and soberness which ought to he employed in discussing it. It seems to me, that nothing can be more admirable than the words of Gregory Nanzianzen: "I cannot think of the unity without being irradiated by the Trinity: I cannot distinguish between the Trinity without being carried up to the unity." (Greg. Nanzian. in Serm. de Sacro Baptis.).Therefore, let us beware of imagining such a Trinity of persons as will distract our thoughts, instead of bringing them instantly back to the unity. The words Father, Son, and Holy Spirit, certainly indicate a real distinction, not allowing us to suppose that they are merely epithets by which God is variously designated from his works. Still they indicate distinction only, not division. The passages we have already quoted show that the Son has a distinct subsistence from the Father, because the Word could not have been with God unless he were distinct from the Father; nor but for this could he have had his glory with the Father. In like manner, Christ distinguishes the Father from himself when he says that there is another who bears witness of him (John 5:32; 8:16). To the same effect is it elsewhere said, that the Father made all things by the Word. This could not be, if he were not in some respect distinct from him. Besides, it was not the Father that descended to the earth, but he who came forth from the Father; nor was it the Father that died and rose again, but he whom the Father had sent. This distinction did not take its beginning at the incarnation: for it is clear that the only begotten Son previously existed in the bosom of the Father (John 1:18). For who will dare to affirm that the Son entered his Father's bosom for the first time, when he came down from heaven to assume human nature? Therefore, he was previously in the bosom of the Father, and had his glory with the Father. Christ intimates the distinction between the Holy Spirit and the Father, when he says that the Spirit proceedeth from the Father, and between the Holy Spirit and himself, when he speaks of him as another as he does when he declares that he will send another Comforter; and in many other passages besides (John 14:6; 15:26; 14:16).
The Institutes of the Christian Religion, Book One, Chapter 13, Sections 14 -- 17: John Calvin, 1583