Marriage has been instituted by God, but it is not a sacrament. Many are the good things which God has instituted, but that does not make them sacraments, which are, by definition, signs and ceremonies to confirm God's promise to us. The fact that marriage illustrates Christ's relationship to the church does not make it a sacrament either - many are the things that illustrate it, but they are not sacraments.

True, Paul calls marriage a 'mystery' (Ephesians 5:32), and says that it speaks of Christ and his church. Says Calvin: 'truly it is a great mystery that Christ allowed a rib to be taken of himself, of which we might be formed; that is, that when he was strong, he was pleased to become weak, that we might be strengthened by his strength, and should no longer live ourselves, but he live in us' (4.19.35).

The problem is that in this passage the word 'mystery' in Greek has been translated by the word 'sacramentum' in Latin, and this has led to the sacramental view of marriage. Calvin responds with some obvious questions. Why debar priests from the 'sacrament'? If marriage is a sacrament, why are the clergy of the Church forbidden to enjoy it? 

Calvin's argument is that the Church's declaration that marriage is a sacrament allowed her to have control over social and civil causes, giving the Church an unwarranted intrusion into issues that are of a personal and individual nature, and in many cases going beyond the biblical parameters within which marriage is to be celebrated and enjoyed.

This recognition of the legitimate area of the Church's authority, as over against the legitimate areas of civil authority, leads Calvin into the final chapter of the Institutes, on the area of civil government. 'He who knows to distinguish between the body and the soul, between the present fleeting life and that which is future and eternal, will have no difficulty in understanding that the spiritual kingdom of Christ and civil government are things very widely separated' (4.20.1).