**EXPOSITION OF THE MORAL LAW.**
In the first place, we must consider what an oath is. An oath, then, is calling God to witness that what we say is true. Execrations being manifestly insulting to God, are unworthy of being classed among oaths. That an oath, when duly taken, is a species of divine worship, appears from many passages of Scripture, as when Isaiah prophesies of the admission of the Assyrians and Egyptians to a participation in the covenant, he says, “In that day shall five cities in the land of Egypt speak the language of Canaan, and swear to the Lord of hosts,” ( Isaiah 19:18 ). Swearing by the name of the Lord here means, that they will make a profession of religion. In like manner, speaking of the extension of the Redeemer’s kingdom, it is said, “He who blesseth himself in the earth shall bless himself in the God of truth: and he that sweareth in the earth shall swear by the God of truth,” ( Isaiah 65:16 ). In Jeremiah it is said, “If they will diligently learn the ways of my people, to swear by my name, The Lord liveth; as they taught my people to swear by Baal; then shall they be built in the midst of my people,” ( Jer. 12:16 ). By appealing to the name of the Lord, and calling him to witness, we are justly said to declare our own religious veneration of him. For we thus acknowledge that he is eternal and unchangeable truth, inasmuch as we not only call upon him, in preference to others, as a fit witness to the truth, but as its only assertor, able to bring hidden things to light, a discerner of the hearts. When human testimony fails, we appeal to God as witness, especially when the matter to be proved lies hid in the conscience. For which reason, the Lord is grievously offended with those who swear by strange gods, and construes such swearing as a proof of open revolt, “Thy children have forsaken me, and sworn by them that are no gods,” ( Jer. 5:7 ). The heinousness of the offence is declared by the punishment denounced against it, “I will cut off them that swear by the Lord, and that swear by Malcham,” ( Zeph. 1:4, 5 ).
Source
source-manifest/institutes— Calvin, Institutes of the Christian Religion, tr. Beveridge 1845 (PD)- evidence_grade: D_doctrinal_textbook