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).