Matthew Henry Commentary Matthew 5:33-37

"Again, ye have heard that it was said to 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; 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 5:33-37 (ASV)

There is no reason to consider solemn oaths in a court of justice, or on other proper occasions, to be wrong, provided they are taken with due reverence. But all oaths taken without necessity, or in common conversation, must be sinful, as well as all those expressions that are appeals to God, even though people may think they can thereby evade the guilt of swearing. The worse people are, the less they are bound by oaths; the better they are, the less need there is for them. Our Lord does not prescribe the precise terms in which we are to affirm or deny, but rather calls for such a constant regard for truth as would make oaths unnecessary.