John Calvin Commentary Matthew 5:37

"But let your speech be, Yea, yea; Nay, nay: and whatsoever is more than these is of the evil [one]." — Matthew 5:37 (ASV)

But your speech shall be, Yes, yes; No, no. Christ now prescribes, in the second place, a remedy, which is that people act towards each other sincerely and honestly, for then simplicity of speech will have quite as much weight as an oath has among those who are not sincere. Now, this is certainly the best way of correcting faults: to point out the sources from which they spring. From where does the great tendency to swear come, if not from the great falsehood, the numerous deceptions, the unsteady and unreliable conduct, so that hardly anything is believed?⁴¹¹ Fairness and honesty in our words are, therefore, demanded by Christ, so that there may no longer be any occasion for an oath.

“Yes, yes; No, no.” This repetition means that we ought to abide by our words, so that all may be convinced of our honesty. Now, as this is the true and lawful method of proceeding, when people have nothing on their tongue but what is in their heart, Christ declares that what is beyond these comes from evil. I do not approve of the interpretation of these words given by some, that the criminality of swearing ought to be attributed to the person who does not believe what another says. Christ teaches us, in my opinion, that the compulsion to swear originates in human wickedness; for if honesty prevailed among people, if they were not inconsistent and hypocritical, they would maintain the simplicity that nature dictates. And yet, it does not follow that it is unlawful to swear when necessity demands it, for many things are proper in themselves, though they have had a wicked origin.

⁴¹¹ “D’ou vient une si grande legerete en sermens, sinon qu'entre tout de mensonges, tromperies, inconstance et babil, on ne sait qui croire, ni a qui se fier?” — “Whence comes so great a lightness about oaths, but that among so many lies and impositions, and so much unsteadiness and trifling, one does not know whom to believe or whom to trust?”?” — “Whence comes so great a lightness about oaths, but that among so many lies and impositions, and so much unsteadiness and trifling, one does not know whom to believe or whom to trust?”