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).

John Calvin on One God, One Faith, and One Baptism

"As God has manifested himself more clearly by the advent of Christ, so he has made himself more familiarly known in the three persons.  Of many proofs let this one suffice.  Paul connects together these three, God, faith, and baptism, and reasons from the one to the other, i.e., because there is one faith he infers that there is one God; and because there is one baptism he infers that there is one faith. Therefore, if by baptism we are initiated into the faith and worship of one God, we must of necessity believe that he into whose name we are baptized is the true God.  And there cannot be a doubt that our Savior wished to testify, by a solemn rehearsal, that the perfect light of faith is now exhibited, when he said, 'Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit' (Matt 28:19), since this is the same thing as to be baptized into the name of the one God, who has been fully manifested in the Father, the Son, and the Spirit.  Hence it plainly appears, that the three persons, in whom alone God is known, subsist in the divine essence.  And since faith certainly ought not to look hither and thither, or run up and down after various objets, but to like, refer, and cleave to God alone, it is obvious that were there various kinds of faith, there behooved also to be various gods.  Then, as the baptism of faith is a sacrament, its unity assures us of the unity of God.  Hence also it is proved that it is lawful only to be baptized into one God, because we make a profession of faith in him in whose name we are baptized.  What, then, is our Savior's meaning in commanding baptism to be administered in the name of the Father, and the Son, and the Holy Spirit, if it be not that we are to believe with one faith in the name of the Father, and the Son, and the Hoy Spirit?  But is this anything else than to declare that the Father, Son, and Spirit, are one God?  Wherefore, since it must be held certain that there is one God, not more than one, we conclude the the Word and Spirit are of the very essence of God" (John Calvin, Institutes of the Christian Religion, 1.13.16).