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 Immensity of God

"The doctrine of Scripture concerning the immensity and the spirituality of the essence of God, should have the effect not only of dissipating the wild dreams of the vulgar, but also of refuting the subtleties of a profane philosophy. . . [God's] immensity surely ought to deter us from measuring him by our sense, while his spiritual nature forbids us to indulge in carnal or earthly speculation concerning him.  With the same view he frequently represents heaven as his dwelling-place.  It is true, indeed, that as he is incomprehensible, he fills the earth also, but knowing that our minds are heavy and grovel on the earth, he raises us above the world, that he make shake off our sluggishness and inactivity.  And here we have a refutation to the error of the Manichees (God is made up of physical body and spiritual soul), who, by adopting two first principles, made the devil almost the equal of God.  This, assuredly, was both to destroy his unity and restrict his immensity.  Their attempt to pervert certain passages of Scripture proved their shameful ignorance, as the very nature of the error did their monstrous infatuation.  The Anthropomorphites (those who attribute human qualities to God), also, who dreamed of a [physical] God, because mouth, ears, eyes, hands, and feet are often ascribed to him in Scripture, are easily refuted.  For who is so devoid of intellect as not to understand that God, in so speaking, lisps with us as nurses are wont to do with little children?  Such modes of expression, therefore, do not so much express what kind of a being God is, as accommodate the knowledge of him to our feebleness.  In doing so he must, of course, stoop far below his proper height" (John Calvin, Institutes of the Christian Religion).