The Anabaptists, not content with this moderation in swearing oaths, condemn all oaths without exception, since Christ’s prohibition of them is general. “I say to you, Do not swear at all, … but let what you say be simply, ‘Yes, yes’ or ‘No, no’; anything more than this comes from evil.” [Matt. 5:34, 37; cf. James 5:12.] But in this way they heedlessly dash against Christ, making him the Father’s enemy as if he had come down to earth to set aside God’s decrees. Now the eternal God not only permits oaths as a legitimate thing under the law (which should be sufficient), but commands their use in case of necessity [Ex. 22:10–11]. But Christ declares that he is one with the Father [John 10:30]; that he brings nothing but what the Father has commanded [John 10:18]; that his teaching is not from himself [John 7:16], etc. What then? Will they make God contradict himself so that he afterward forbids and condemns what he once approved by enjoining it upon men’s behavior?
But because there is some difficulty in Christ’s words, let us spend a little time on them. Here, however, we shall never attain the truth unless we fix our eyes upon Christ’s intention and give heed to what he is driving at in that passage. It was not his purpose either to slacken or tighten the law, but to bring back to a true and genuine understanding what had been quite corrupted by the false devisings of the scribes and Pharisees. If we understand this, we will not think that Christ condemned oaths entirely, but only those which transgress the rule of the law. From these words it is clear that the people then commonly avoided perjury only, while the law forbids not only perjuries but also empty and superfluous oaths. Therefore the Lord, the surest interpreter of the law, warns that it is evil not only to swear falsely but also to swear [Matt. 5:34]. Why “to swear”? Surely he means “to swear in vain.” But the oaths that are commended in the law, he leaves untouched and free. Our opponents think that they argue more compellingly when they doggedly seize upon the expression “at all.” Yet this does not refer to the word “to swear,” but to the forms of oaths following thereafter. For this, also, was a part of their error, that while they swore by heaven and earth they thought they did not touch the name of God. After the chief instance of transgression, therefore, the Lord also cuts off all excuses from them in order that they may not suppose they have escaped by calling on heaven and earth, while suppressing God’s name. eWe ought also to note this in passing: although the name of God is not expressed, yet men swear by him in indirect forms; as when they swear by the light of life, by the bread they eat, by their baptism, or by other tokens of God’s generosity toward them. Christ, in that passage forbidding men to swear by heaven and earth and Jerusalem [Matt. 5:34–35], is not correcting superstition, as some falsely think. Rather, he is refuting the wily sophistry of those who see nothing wrong in idly tossing about indirect oaths—as if they spared God’s sacred name, when it is actually engraved upon all his benefits. It is another matter when some mortal, or deceased person, or angel is substituted in place of God; just as among the heathen nations that loathsome form of swearing by the life or by the genius of the king was devised by way of adulation. For then such false deification obscures and lessens the glory of the one God. But when we intend only to seek confirmation of our statements from God’s holy name, although it be done indirectly, injury is done to his majesty by all such trifling oaths. Christ deprives this license of vain excuse, forbidding us to “swear at all.” James, repeating those words of Christ which I have cited, has the same intent [James 5:12]. For such rashness, although it is a desecration of God’s name, has always been widespread in the world. If you should refer the expression “at all” to the substance, as if it were without exception unlawful to swear any oath, how would you explain what is immediately added: “Neither by heaven, nor by the earth,” etc.? From these words it is sufficiently clear that Christ has met the quibbles whereby the Jews thought their fault lightened.
—John Calvin, Institutes of the Christian Religion ed. John T. McNeill, trans. Ford Lewis Battles. The Library of Christian Classics (Louisville: Westminster John Knox Press, 2011), 2.8.26.