Objection: 1. But Christ says, “Swear not at all;” and James says, “Nor by any other oath…”. Therefore Christians are not allowed to swear [oaths] under any form.
Answer: There is here a fallacy of composition; for when Christ says, swear not at all, we are not to refer this language to the oath itself, but to the various forms of rash [oath] swearing which the Pharisees imagined lawful. It is, therefore, as if he would say, “Swear not falsely or rashly at all, whether it be in a direct or indirect way.” So when the Apostle James says, “Nor by any other oath,” we must understand him also as referring to such oaths as are rash and false, of which kind he furnishes some specimens, and forbids all of a similar character. If this be not the proper interpretation of these passages, Christ himself has violated his own precept which he here lays down, saying, “Let your communication be yes, yes or no, no;” for he frequently in his discourses used this most emphatic form of expression, “Truly, truly I say to you.” And James would in this case condemn Paul, who called God for a record upon his soul. And the Holy Spirit would contradict himself by condemning all oaths by James, and commending them by another Apostle as a remedy useful and necessary to the preservation of society, for the purpose of putting an end to strife and controversies, from which human life, in this state of frailty and imperfection, cannot be free.
Objection 2: But such oaths as were permitted, together with the examples which are found in the Scriptures, have respect to public oaths—such as were exacted or given in the name of the public and for the public good. Therefore at least private oaths, or such as pass between private individuals, are entirely prohibited.
Answer. 1: We deny the antecedent; because there is not only no such restriction as that which is here maintained, specified in the instances recorded in the Scriptures, where the saints make oath to God, but it is impossible to interpret them in this way, as a careful examination of the passages themselves will prove.
2: There are many oaths recorded in the Scriptures, the private character of which cannot be doubted, such as that of Jacob and Laban, that of Boaz, Abdiah, Abigail, and David. (Gen. 31:53. Ruth 3:13. etc.) 3. The same thing may be proven from the design of the oath, which is a confirmation of fidelity and truth among men, and the putting an end to strife. These things now have respect to Christians also as private individuals; and hence the oath itself by which we establish truth and fidelity, likewise has respect to them.
—Zacharias Ursinus, Body of Orthodox Doctrine, on Heidelberg Catechism 101.