Trinitarian Proof

      "As God has shown himself more clearly in the coming of Christ, so he has made himself more widely understood as three persons. Of the many proofs, let this one be enough. Paul connects God, faith and baptism and reasons from one to another (Eph. 4:5). So because there is one faith, he infers that there is one God; and because there is one baptism, he infers there is one faith. Therefore, if by baptism we are initiated into the faith and worship of one God, we must, of course, believe that the one into whose name we are baptised is the true God. And there can be no doubt that our Saviour wanted to proclaim, in a most positive statement, that the full light of faith can be seen, when he said 'Therefore go and make disciples of all nations, baptising them in the name of the Father and of the Son and of the Holy Spirit' (Matt. 28:19).
      "Since this is the same as being baptised into the name of the one God, who has been shown forth in the Father, the Son and the Holy Spirit, so it is obvious that the three persons, by whom God is known, coexist in the divine nature. Because faith ought not to be dashing around in all directions but should hold fast to God alone, it is obvious that if there were varieties of faith, there would have to be a variety of gods. So, because baptism is a sacrament, its unity assures us of God's unity. Moreover, it is self-evident that we must only be baptised into one God, because we make profession of faith in him in whose name we are baptised. So what does our Saviour mean, when he commands that baptism should be administered in the name of the Father, and the Son, and the Holy Spirit, if it is not that we are to be united in one faith in them? This is the same as stating that the Trinity is one God. So since we must believe that there is one God, and not more than one, we conclude that the Word and Spirit are of the very essence of God..."

John Calvin, The Institutes of Christian Religion (Baker Academic, 1987), p. 52-53.