Thou shalt not perjure thyself This also is not a correction of the law, but a true interpretation of it. For God condemned in the law not only acts of perjury, but lightness in swearing, which lessens the reverence for his name. The man who perjures himself is not the only person who takes the name of God in vain, (Exodus 20:7.) He does so, who idly and contemptuously pronounces the name of God on trivial occasions, or in ordinary conversation. While the law condemns every kind of profanation of the name of God the Jews imagined, that the guilt of it lay entirely in acts of perjury. Christ reproves this gross error of supposing that they might, without danger, abuse the name of God, provided they did not swear falsely. We are, no doubt, strictly enjoined to perform to the Lord what we have sworn: for he who, after employing the name of God, cheats and deceives his neighbors, does an injury to God as well as to man. But it is improper to confine to a single part that which has a wider reference. Some consider the word perform as applying to vows, when any thing has been promised to God on account of religion. But this mode of expression applies very well to all promises and engagements, which have been sanctioned by the use of the name of God: for in such cases God is appealed to as guarantee between the parties, to secure their fidelity.

Thou shalt not forswear thyself - Christ here proceeds to correct another false interpretation of the law. The law respecting oaths is found in Leviticus 19:12, and Deuteronomy 23:23. By those laws people were forbid to perjure themselves, or to forswear, that is, swear falsely.
Perform unto the Lord - Perform literally, really, and religiously what is promised in an oath.
Thine oaths - An oath is a solemn affirmation or declaration, made with an appeal to God for the truth of what is affirmed, and imprecating his vengeance, and renouncing his favor if what is affirmed is false. A false oath is called perjury, or, as in this place, forswearing.
It appears, however, from this passage, as well as from the ancient writings of the Jewish rabbins, that while the Jews professedly adhered to the law, they had introduced a number of oaths in common conversation, and oaths which they by no means considered to be binding. For example, they would swear by the temple, by the head, by heaven, by the earth. So long as they kept from swearing by the name Yahweh, and so long as they observed the oaths publicly taken, they seemed to consider all others as allowable, and allowedly broken. This is the abuse which Christ wished to correct. "It was the practice of swearing in common conversation, and especially swearing by created things." To do this, he said that they were mistaken in their views of the sacredness of such oaths. They were very closely connected with God; and to trifle with them was a species of trifling with God. Heaven is his throne; the earth his footstool; Jerusalem his special abode; the head was made by him, and was so much under his control that we could not make one hair white or black. To swear by these things, therefore, was to treat irreverently objects created by God, and could not be without guilt. It is remarkable that the sin here condemned by the Saviour prevails still in Palestine in the same form and manner referred to here. Dr. Thomson (The Land and the Book, vol. ii. p. 284) says, "The people now use the very same sort of oaths that are mentioned and condemned by our Lord. They swear by the head, by their life, by heaven, and by the temple, or what is in its place, the church. The forms of cursing and swearing, however, are almost infinite, and fall on the pained ear all day long."
Our Saviour here evidently had no reference to judicial oaths, or oaths taken in a court of justice. It was merely the foolish and wicked habit of swearing in private conversation; of swearing on every occasion and by everything that he condemned. This he does condemn in a most unqualified manner. He himself, however, did not refuse to take an oath in a court of law, Matthew 26:63-64. So Paul often called God to witness his sincerity, which is all that is meant by an oath. See Romans 1:9; Romans 9:1; Galatians 1:20; Hebrews 6:16. Oaths were, moreover, prescribed in the law of Moses, and Christ did not come to repeal those laws. See Exodus 22:11; Leviticus 5:1; Numbers 5:19; Deuteronomy 29:12, Deuteronomy 29:14.

Thou shalt not forswear thyself - They dishonor the great God, and break this commandment, who use frequent oaths and imprecations, even in reference to things that are true; and those who make vows and promises, which they either cannot perform, or do not design to fulfill, are not less criminal. Swearing in civil matters is become so frequent, that the dread and obligation of an oath are utterly lost in it. In certain places, where oaths are frequently administered, people have been known to kiss their thumb or pen, instead of the book, thinking thereby to avoid the sin of perjury; but this is a shocking imposition on their own souls. See the notes on Deuteronomy 4:26; Deuteronomy 6:13.
Perform unto the Lord thine oaths - The morality of the Jews on this point was truly execrable: they maintained, that a man might swear with his lips, and annul it in the same moment in his heart. Rab. Akiba is quoted as an example of this kind of swearing. See Schoettgen.

(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.

Again, ye have heard that it hath been said,.... Besides what has been observed, in ver. 21 and 27 you know it has also been said,
by, or to them of old time, what is written in Leviticus 19:12. "And ye shall not swear by my name falsely"; which seems to be referred to, when it is said, "thou shalt not forswear thyself": and is the law forbidding perjury, or false swearing; and was what the Jews were chiefly, if not only concerned about; little regarding the vanity, only the truth of an oath: for they took swearing vainly, to be the same as swearing falsely; wherefore so long as what they swore was truth, they were not careful whether it was of any importance or not: moreover, these men sinned, in that they swore by the creatures, which they thought they might do, and not sin; and when they had so done, were not under obligation to perform; because they made no use of the name of God, to whom only vows and oaths were to be performed, "but shalt perform unto the Lord thine oaths", Numbers 30:2 which they understood of vows only made to the Lord, and not to others; and of oaths, when in his name, and not by others; which they did do, and yet thought themselves not obliged by them.

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.

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 Exodus 20:7; but they express all that it was currently understood to condemn, namely, false swearing (Leviticus 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 Isaiah 66:1);

Thou shalt not forswear thyself. (See Leviticus 19:12; and Numbers 25:2.) The Jews held that this only prohibited swearing falsely and by the name of God.

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. Exodus 20:7.