I  confess I am not entirely certain of the full extent of Christ’s warning concerning vows, but it seems He is establishing a principle of carefulness and honesty for His people.  It does not seem appropriate to think that He is overturning altogether the very idea of vows, or taking an oath under any circumstance at all, although some have certainly taken it that way.  But the Lord in the Old Testament permitted and regulated the offering of vows, and there seem to be very excellent principles behind the practice of witnesses in legal proceedings taking an oath, in order to guard against perjury.  These things, then, should not be regarded as sinful in themselves.

Without becoming involved in a deep study of the matter, it seems to me Christ is warning against a reckless rashness in making vows or oaths.  In our common conversation, we ought not to be constantly promising, swearing, and taking an oath to confirm every word we speak.  These things ought never to be done except with the greatest solemnity, and with firm purpose.  But in general, our conduct and our speech ought to be such that we may be taken at our word.  Honesty ought to be one of the glaring virtues of Christ’s people.  When we give our yea or our nay, people should be able to take us at our words.  If we are having to affix an “I swear” to our statements, then it betrays a dishonesty of character that gives no praise to the character of the professed servant of Christ.