33 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: 34 But I say unto you, Swear not at all; neither by heaven; for it is God’s throne: 35 Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. 36 Neither shalt thou swear by thy head, because thou canst not make one hair white or black. (Matthew 5:33-36 KJV)
William Burkitt’s Commentary
The next commandment which our Saviour expounds and vindicates, is the third, which requires a reverent use of God’s name.
Now the Pharisees taught that perjury was the only breach of this commandment; and that swearing was nothing, if they did not forswear themselves; and that persons were only obliged to swear by the name of God in public courts of justice, but in their ordinary and common discourse they might swear by any of the creatures.
Now, in opposition to these wicked principles and practices, Christ says, Swear not at all: that is,
1. Swear not profanely in your ordinary discourse.
2. Swear not unduly by any of the creatures; for that is to ascribe a deity to them.
3. Swear not lightly upon any trifling or frivolous occasion; for oaths upon small occasions are great sins. So that an oath is not here forbidden by our Saviour, but restrained.
For though light and needless, common and ordinary swearing, be a very great sin, yet to take an oath upon a solemn occasion, when adjured by the high-priest, did answer upon oath. But he forbids all voluntary oaths in common conversation, and in our ordinary discourse; because an oath is an act of religious worship; therefore to trifle with it is an horrid provocation.