Commenting on the torah’s limited allowance of divorce, Calvin writes:
As the Jews falsely imagined that they discharged their whole duty toward God, when they kept the law in a national manner, so whatever the national law did not forbid, they foolishly supposed to be lawful. Divorces, which husbands were wont to give to their wives, had not been prohibited by Moses as to external order, but only, for the sake of restraining lewdness, he had ordered that “a bill of divorcement” should be given to the wives who were put away, (Deuteronomy 24:1.) It was a sort of testimonial of freedom, so that the woman was afterwards free from the yoke and power of the husband; while the husband at the same time acknowledged, that he did not send her away on account of any crime, but because she did not please him. Hence proceeded the error, that there was nothing wrong in such putting away, provided that the forms of law were observed.
But they did wrong in viewing as a matter of civil law, the rule which had been given them for a devout and holy life. For national laws are sometimes accommodated to the manners of men but God, in prescribing a spiritual law, looked not at what men can do, but at what they ought to do. It contains a perfect and entire righteousness, though we want ability to fulfill it. Christ, therefore, admonishes us not to conclude, that what is allowed by the national law of Moses is, on that account, lawful in the sight of God. That man, (says he,) who puts away his wife, and gives her a bill of divorcement, shelters himself under the pretense of the law: but the bond of marriage is too sacred to be dissolved at the will, or rather at the licentious pleasure, of men. Though the husband and the wife are united by mutual consent, yet God binds them by an indissoluble tie, so that they are not afterwards at liberty to separate. An exception is added, except on account of fornication:for the woman, who has basely violated the marriage-vow, is justly cast off; because it was by her fault that the tie was broken, and the husband set at liberty.
(Comment. on Matt. 5:31-32; Luke 16:18)
Jesus’ words in these verses are widely debated among exegetes today, but Calvin’s philosophy of law nevertheless comes out in his commentary. He states that “the national law of Moses” is not identical with the moral law (simpliciter) nor with the “spiritual law” of what God expects of righteous men. Civil laws are, instead, “sometimes accommodated to the manners of men.” This need not be interpreted to mean that the law in question was an unjust law, but rather that it was limited according to the public and measurable abilities of man. Therefore, something being lawful is quite distinct from it being morally admirable.
This is one reason why the Mosaic law need not be a standard for all other civil laws in history. Times and occasions may change political efficiency. It is also an explanation for why not all sins need civil legislation. Some forms of divorce may be civilly permissible, though still immoral, whereas other forms may need to be legally prohibited.