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).