{8} Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths:

(8) The meaning of the third commandment against the perverse opinion and judgment of the scribes, who excused by oaths or indirect forms of swearing.

People's New Testament

5:33 Thou shalt not forswear thyself. See Le 19:12 Nu 25:2. The Jews held that this only prohibited swearing falsely and by the name of God.

Wesley's Notes

5:33 Our Lord here refers to the promise made to the pure in heart of seeing God in all things, and points out a false doctrine of the scribes, which arose from their not thus seeing God. What he forbids is, the swearing at all, by any creature, in our ordinary conversation: both of which the scribes and Pharisees taught to be perfectly innocent. Exod 20:7.

Jamieson-Fausset-Brown Bible Commentary

33. Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself-These are not the precise words of Ex 20:7; but they express all that it was currently understood to condemn, namely, false swearing (Le 19:12, &c.). This is plain from what follows.

But I say unto you, Swear not at all-That this was meant to condemn swearing of every kind and on every occasion-as the Society of Friends and some other ultra-moralists allege-is not for a moment to be thought. For even Jehovah is said once and again to have sworn by Himself; and our Lord certainly answered upon oath to a question put to Him by the high priest; and the apostle several times, and in the most solemn language, takes God to witness that he spoke and wrote the truth; and it is inconceivable that our Lord should here have quoted the precept about not forswearing ourselves, but performing to the Lord our oaths, only to give a precept of His own directly in the teeth of it. Evidently, it is swearing in common intercourse and on frivolous occasions that is here meant. Frivolous oaths were indeed severely condemned in the teaching of the times. But so narrow was the circle of them that a man might swear, says Lightfoot, a hundred thousand times and yet not be guilty of vain swearing. Hardly anything was regarded as an oath if only the name of God were not in it; just as among ourselves, as Trench well remarks, a certain lingering reverence for the name of God leads to cutting off portions of His name, or uttering sounds nearly resembling it, or substituting the name of some heathen deity, in profane exclamations or asseverations. Against all this our Lord now speaks decisively; teaching His audience that every oath carries an appeal to God, whether named or not.

neither by heaven; for it is God's throne-(quoting Isa 66:1);

Matthew Henry's Concise Commentary

5:33-37 There is no reason to consider that solemn oaths in a court of justice, or on other proper occasions, are 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 which are appeals to God, though persons think thereby to evade the guilt of swearing. The worse men are, the less they are bound by oaths; the better they are, the less there is need for them. Our Lord does not enjoin the precise terms wherein we are to affirm or deny, but such a constant regard to truth as would render oaths unnecessary.