Our Lord is forbidding here the flippant, profane, or careless use of oaths in everyday speech in that culture, such oaths were often employed for deceptive purposes. Those doing that would swear by “heaven,” “earth,” “Jerusalem,” or their own “heads” (vv. 34-36), not by God. In this they hoped to avoid divine judgment for their lie, however, all of it is in God’s creation, so it drew him in, and produced guilt before him just as if the oath were made in his name. Jesus suggested that all our speech should be as if we were under an oath to tell the truth (v37).
12 You shall not swear falsely by My name, so as to profane the name of your God; I am the LORD. Leviticus 19:12 (NASB)
2 If a man makes a vow to the LORD, or takes an oath to bind himself with a binding obligation, he shall not violate his word; he shall do according to all that proceeds out of his mouth. Numbers 30:2 (NASB)
21 “When you make a vow to the LORD your God, you shall not delay to pay it, for it would be sin in you, and the LORD your God will surely require it of you. 22 However, if you refrain from vowing, it would not be sin in you. 23 You shall be careful to perform what goes out from your lips, just as you have voluntarily vowed to the LORD your God, what you have promised. Deuteronomy 23:21-23 (NASB)
33 “Again, you have heard that the ancients were told, ‘ YOU SHALL NOT MAKE FALSE VOWS, BUT SHALL FULFILL YOUR VOWS TO THE LORD.’ 34 But I say to you, make no oath at all, either by heaven, for it is the throne of God, 35 or by the earth, for it is the footstool of His feet, or by Jerusalem, for it is THE CITY OF THE GREAT KING. 36 Nor shall you make an oath by your head, for you cannot make one hair white or black. 37 But let your statement be, ‘Yes, yes’ or ‘No, no’; anything beyond these is of evil. Matthew 5:33-37 (NASB)
For a short period of time in late 1980’s I worked in a PC/Computer store in sales. I am not a sales person, but most if not all of the other fellows I worked with were. Instead, I simply told people what the computers could do or what they could not do and tried to match them up with what the people needed. I was usually in the top two or three in sales, never number one there, but I was always number one in customer satisfaction. In fact, the only time I ever saw those customers again was if something broke or they wanted an upgrade or they brought a friend or relative in to buy a computer. However, something changed when I sold a truckload of computers to a local school. Then I did it again.