Matthew 5:33

The law had prescribed a penalty for false swearing, so that the ritual of an oath might hold false testimony in check.… But faith removes the need for using an oath. It establishes in truth the dealings of our life. Once the inclination to deceive has been checked, it enjoins simplicity in speaking and hearing.… Therefore those who are living in the simplicity of faith have no need for the ritual of an oath. With such people, what is, always is, and what is not, is not. For this reason, their every word and deed are always truthful.

"Again, ye have heard that it was said to them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths. But I say unto you, swear not at all." Why did He go straightway not to theft, but to false witness, passing over that commandment? Because he that steals, doth upon occasion swear also; but he that knows not either swearing or speaking falsehood, much less will he choose to steal. So that by this He hath overthrown the other sin likewise: since falsehood comes of stealing. But what means, "Thou shalt perform unto the Lord thine oaths?" It is this, "thou shalt be true in swearing." "But I say unto you, swear not at all." Next, to lead them farther away from swearing by God, He saith, "Neither by Heaven, for it is God's throne, nor by the earth, for it is the footstool of His feet; nor by Jerusalem, for it is the city of the great King:" still speaking out of the prophetical writings, and signifying Himself not to be opposed to the ancients. This was because they had a custom of swearing by these objects, and he intimates this custom near the end of his Gospel. But mark, I pray thee, on what ground He magnifies the elements; not from their own nature, but from God's relation to them, such as it had been in condescension declared. For because the tyranny of idolatry was great, that the elements might not be thought worthy of honor for their own sake, He hath assigned this cause, which we have mentioned, which again would pass on to the glory of God. For He neither said, "because Heaven is beautiful and great," nor, "because earth is profitable;" but "because the one is God's throne, the other His footstool;" on every side urging them on towards their Lord.

(cont. Faust. xix. 23.) Inasmuch as the sin of perjury is a grievous sin, he must be further removed from it who uses no oath, than he who is ready to swear on every occasion, and the Lord would rather that we should not swear and keep close to the truth, than that swearing we should come near to perjury. (Serm. in Mont. i. 17.) This precept also confirms the righteousness of the Pharisees, not to forswear; inasmuch as he who swears not at all cannot forswear himself. But as to call God to witness is to swear, does not the Apostle break this commandment when he says several times to the Galatians, The things which I write unto you, behold, before God, I lie not. (Gal. 1:20.) So the Romans, God is my witness, whom I serve in my spirit. (Rom. 1:9) Unless perhaps some one may say, it is no oath unless I use the form of swearing by some object; and that the Apostle did not swear in saying, God is my witness. It is ridiculous to make such a distinction; yet the Apostle has used even this form, I die daily, by your boasting. (1 Cor. 15:31.) That this does not mean, your boasting has caused my dying daily, but is an oath, is clear from the Greek, which is νὴ τὴν ὑμετέραν καύχησιν.

(non occ.) The Lord has hitherto taught to abstain from injuring our neighbour, forbidding anger with murder, lust with adultery, and the putting away a wife with a bill of divorce. He now proceeds to teach to abstain from injury to God, forbidding not only perjury as an evil in itself, but even all oaths as the cause of evil, saying, Ye have heard it said by them of old, Thou shall not forswear thyself. It is written in Leviticus, Thou shalt not forswear thyself in my name; (c. 19:12.) and that they should not make gods of the creature, they are commanded to render to God their oaths, and not to swear by any creature, Render to the Lord thy oaths; that is, if you shall have occasion to swear, you shall swear by the Creator and not by the creature. As it is written in Deuteronomy, Thou shall fear the Lord thy God, and shall swear by his name. (c. 6:13.)

Again, ye have heard that it hath been said, Thou shalt not swear falsely, but shalt perform unto the Lord thine oaths. That is, when you take an oath, speak the truth.