John Calvin on the Unity and Distinction of the Trinity

"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 be employed in discussing it.  It seems to me, that nothing can be more admirable than the words of Gregory Nazianzen: '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.'  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 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 proceeds 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)" (John Calvin, Institutes of the Christian Religion, 1.13.17).