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.