I. Our Lord does not here forbid a solemn oath, such as may be required, for example, in a court of justice. The law clearly allowed such an oath for confirmation. When the high priest adjured Jesus by the living God, Jesus found no fault with it. And therefore it seems to me the act, not of an enlightened, but of an over-scrupulous conscience to refuse an oath in such circumstances. Christ does not allude to solemn adjurations, but only to the flippant expletives which were and are so eagerly used, in such a way as to impair the perfect simplicity and veracity of men's souls.
II. Our Lord here obviously forbids all profane swearing. For other sins, it has been said, one may have something to show. But in the case of a profane swearer a man sells his soul absolutely for nought. It is the veriest wilful and wanton outrage of God's law, without advantage to the sinner himself, and most revolting to every well-constituted mind. It is an utterly profitless vice, a degrading of God's good gift of speech, without reason and without excuse. Therefore, "Let your yea be yea, and your nay, nay: for whatsoever is more than these cometh of evil."
III. I apprehend that our Lord's special object here is to insist on His people practising the habit of absolute truthfulness, which will not need any oath to confirm it, and which is apt to be greatly weakened by the use of such language. The needless taking of oaths tends to lessen a man's sense of truth, and enfeeble his regard for it. Men who swear much by heaven or by earth do not regard such oaths as very binding; and once they have accustomed themselves to untruth in this way, bigger and rounder adjurations will be needed, and will be found equally useless, until the whole soul becomes corrupted with that worst of all rottenness an utterly lying spirit.
W. C. Smith, The Sermon on the Mount,p. 116.
References: Matthew 5:33-37. J. Oswald Dykes, The Laws of the Kingdom,p. 65; Ibid., The Manifesto of the King,p. 265.Matthew 5:33-48. Parker, Inner Life of Christ,vol. i., p. 193.Matthew 5:34. J. N. Norton, The King's Ferry Boat,p. 146; R. Newton, Bible Warnings,p. 334.