In this one essence are three persons, yet so that neither is there a triple God, nor is the simple essence of God divided. Meaning of the word Person in this discussion. Three hypostases in God, or the essence of God.

2. But there is another special mark by which he designates himself, for the purpose of giving a more intimate knowledge of his nature. While he 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 inveigh against the term Person as being merely of human inventions 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,” (Hebrews 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 diminution, it is improper, nay, ridiculous, to call it his express image, (charaktes.) 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, viz., 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 refulgent 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 impious to multiply. If credit, then, is given to the Apostle’s testimony, it follows that there are three persons (hypostases) in God. The Latins having used the word Persona to express the same thing as the Greek “hupostatis”, it betrays excessive fastidiousness and even perverseness to quarrel with the term. The most literal translation would be subsistence. Many have used substance in the same sense. Nor, indeed, was the use of the term Person confined to the Latin Church. For the Greek Church in like manner, perhaps, for the purpose of testifying their consent, have taught that there are three “prosopa” (aspects) in God. All these, however, whether Greeks or Latins, though differing as to the word, are perfectly agreed in substance.

John Calvin-Institutes of the Christian Religion-Book I-Chapter 13-Henry Beveridge Translation