Charles Spurgeon Commentary Matthew 5:34-37

"but I say unto you, swear not at all; neither by the heaven, for it is the throne of God; nor by the earth, for it is the footstool of his feet; nor by Jerusalem, for it is the city of the great King. Neither shalt thou swear by thy head, for thou canst not make one hair white or black. But let your speech be, Yea, yea; Nay, nay: and whatsoever is more than these is of the evil [one]." — Matthew 5:34-37 (ASV)

Neither by heaven; for it is God's throne: Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.

If words mean anything, this command of Christ is an utter abolishment of oaths taken before magistrates as well as everywhere else. I can make nothing else out of it; indeed, it must mean that, because Christ contrasts his teaching with that of former ages: It hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: but I say unto you, Swear not at all. A man who cannot be believed upon his word certainly cannot be believed upon his oath; and, usually, when a man tells a lie, the next thing he does is to swear to it.

When Peter denied his Master, the next thing he did was to curse and to swear, because he thought it likely that they would not imagine that he was a follower of Christ if he did curse and swear; so he gave that as a pretty clear proof that he had not been with Christ, and was not one of his disciples. Alas, that we should need anything beside Yea, yea, and Nay, nay!