John Calvin on the Three Persons of God

"While [God] proclaims his unity, he distinctly sets it before us as existing in three persons.  These we must hold, unless the bare and empty name of Deity merely is to flutter in our brain without any genuine knowledge.  Moreover, lest any one should dream of a threefold God, or think that the simple essence is divided by the three persons, we must here seek a brief and easy definition which may effectually guard us from error.  But as some strongly [rail] against the term person as being merely of human invention, let us first consider how far they have any ground for doing so.  When the apostle calls the Son of God 'the express image of his person' (Her 1:3), he undoubtedly does assign to the Father some subsistence in which he differs from the Son.  For to hold with some interpreters that the term is equivalent to essence (as if Christ represented the substance of the Father like the impression of a seal upon wax), were not only harsh but absurd.  For the essence of God being simple and undivided, and contained in himself entire, in full perfection, without partition or [being diminished], it is improper, no ridiculous, to call it his express image χαρακτήρ (character).  But because the Father, though distinguished by his own peculiar properties, has expressed himself wholly in the Son, he is said with perfect reason to have rendered his person (hypostasis) manifest in him.  And this aptly accords with what is immediately added, i.e., that he is 'the brightness of his glory.'  The fair inference from the apostle's words is, that there is a proper subsistence (hypostasis) of the Father, which shines [radiantly] in the Son.  From this, again it is easy to infer that there is a subsistence (hypostasis) of the Son which distinguishes him from the Father.  The same holds in the case of the Holy Spirit; for we will immediately prove both that he is God, and that he has a separate subsistence from the Father.  This, moreover, is not a distinction of essence, which it were [lacking reverence for God] to multiply.  If credit, then, is given to the apostle's testimony, it follows that there are three persons (hypostases) in God" (John Calvin, Institutes of the Christian Religion).

John Calvin on the Holy Spirit and Scripture

"It is easy to understand that we must give diligent heed both to the reading and hearing of Scripture, if we would obtain any benefit from the Spirit of God (just as Peter praises those who attentively study the doctrine of the prophets [2 Pet 1:19], though it might have been thought to be superseded after the gospel light arose), and, on the contrary, that any spirit which passes by the wisdom of God's word, and suggests any other doctrine, is deservedly suspected of vanity and falsehood.  Since Satan transforms himself into an angel of light, what authority can the Spirit have with us if he be not ascertained by an infallible mark?  And assuredly he is pointed out to us by the Lord with sufficient clearness; but these miserable men err as if bent on their own destruction, while they seek the Spirit from themselves rather than from him.  But they say that it is insulting to subject the Spirit, to whom all things are to be subject, to the Scripture; as if it were disgraceful to the Holy Spirit to maintain a perfect resemblance throughout, and be in all respects without variation consistent with himself.  True, if he were subjected to a human, an angelical, or to any foreign standard, it might be thought that he was rendered subordinate, or, if you will, brought into bondage, but so long as he is compared with himself, and considered in himself, how can it be said that he is thereby injured?  I admit that he is brought to a test, but the very test by which it has pleased him that his majesty should be confirmed.  It ought to be enough for us when once we hear his voice; but lest Satan should insinuate himself under his name, he wishes us to recognize him by the image which he has stamped on the Scriptures. The author of the Scriptures cannot vary, and change his likeness.  Such as he there appeared at first, such he will perpetually remain.  There is nothing [humiliating] to him in this, unless we are to think it would be honorable to for him to degenerate, and revolt against himself" (Institutes of Christian Religion, John Calvin).