Among the Jews, it had become common to resort to oaths in daily life. This is reflected in the admonition contained in the book of Sirach (written in Hebrew early in the second century BCE and translated into Greek by the writer’s grandson after 132 BCE). The writer included the admonition: “Let not your mouth form the habit of swearing, or becoming too familiar with the Holy Name. Just as a slave that is constantly under scrutiny will not be without welts, so one who swears continually by the Holy Name will not remain free from sin. A man who swears often heaps up obligations.” (Sirach 23:9-11, NAB)

The Pharisees established a gradation for oaths, setting aside the binding nature of certain formulas. According to ancient Jewish sources, swearing by heaven, the earth or the sun was not considered to be an oath even if the intent of the individual had been to swear by the Creator. It was customary to swear by various things, including Jerusalem, the temple, the altar, sacrifices, and the life of one’s head. One prominent rabbi (Judah) is quoted in the Tosefta as saying: “He who says, ‘By Jerusalem,’ says nothing, unless with an intent purpose he shall vow toward Jerusalem.”

So it must have been common knowledge that it had been said to those of old, “You must not swear falsely, but you must pay your vows to the Lord.” Jesus, however, directed his comments against the practice of resorting to the use of oaths to add credibility to statements or promises. “Do not swear at all,” he said and then pointed out that, even though not mentioned in the particular formula being used, God was involved when swearing by heaven, the earth, or Jerusalem. This is so because heaven is God’s throne, the earth is his footstool, and Jerusalem is his city. The Most High is the Sovereign and, therefore, Jesus referred to Jerusalem as the “city of the great king.” (Matthew 5:33-35)

Presenting the reason for not swearing by one’s head, the Son of God said, “You cannot make one hair white or black,” showing that humans have no real control of their life even in insignificant matters. Therefore, instead of swearing to assure others they were speaking the truth or excusing nonfulfillment of a promise or agreement on the basis that a particular formula used in swearing did not impose a binding obligation, all who heed Jesus’ teaching should be dependable respecting their word, letting their “Yes” mean “Yes” and their “No” mean “No.” (Matthew 5:36, 37) He identified whatever went beyond not maintaining trustworthiness in word as being from the wicked one, the “devil” or “slanderer,” with whom lying had its start. (Compare John 8:44.)