Calvin has already established his understanding of "a twofold government" to which human beings are subject: an inward government in which God rules over the individual human soul for eternal life and an outward government in which God through human government establishes civil justice and outward morality (4.20.1).
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.
Walk prudently when you go to the house of God;
and draw near to hear rather than to give the sacrifice of fools,
for they do not know that they do evil. Do not be rash with your mouth,
And let not your heart utter anything hastily before God.
For God is in heaven, and you on earth;
Therefore let your words be few.
~ Ecclesiastes 5:1-2~
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