Oath taking was common in Jesus day, since it was permissible by the Mosaic Law (Deut 23:21-23). An oath was pronouncing a curse upon one’s self if the truth was not spoken, or a promise is not fulfilled. The first century Jewish philosopher Philo defined an oath as an “appeal to God as a witness on matters in dispute, and to call him as a witness to a lie is the height of profanity” (Spec. Leg. 2.10). This was an important issue in lawsuits and daily life since it underscored the truthfulness of one’s statement. The oath was only required where truthfulness was in doubt, possibly in a legal situation. The Law allowed for oath making but emphasized the importance of keeping an oath. If you promise something you are to keep that promise.
Traditionally one would “swear by” something. Swearing by the name of God (or a god) was common (Exod 20:7, Lev 19:21), but God did not want his people using his name for oaths they did not intend to keep. Swearing by the King’s name was possible since the king in Israel was to be God’s representative. One might swear by his own life that his word was true, something like “swearing by my head.” Swearing by “heaven and earth” was also common, as was swearing by the angels, the temple, or parts of the temple. Oaths sworn on things other than God were “less binding,” they were less serious (Matt 23:16).
Jesus is reacting against those persons who invoke God by oath and do not fulfill that oath. By swearing an oath the hypocritical Pharisee would imply that he was truthful. But then he would say later than an oath made on the temple isn’t binding. This is the issue in Mt. 23 16-20. It is the equivalent to a child saying “had my fingers crossed.”
As with each other extensions of the Law in the Sermon on the Mount, Jesus’ ideal is an absolute virtue: Do not swear oaths you cannot or don’t intend to keep. If you cannot keep an oath, it is better to not swear oaths at all! The problem Jesus is getting at is that if your word cannot be trusted then you are nearly worthless to the kingdom of Heaven.
In contrast to making an oath you may not keep, Jesus says: Let your “yes” be “yes”, your “no” be “no”. This simply means tell the truth, and faithfully do what you promise. Like the rest of the Sermon on the Mount, Jesus is extreme and hyperbolic – does he really mean his people ought never swear an oath? Like self-mutilation in the adultery passage, Jesus’ extreme statement emphasizes the heart of the original command: be honest and keep your promises!
This is a radical concept in our day as much as it was in Jesus day. He is commanding that we live by the words we say! If we promise it, we have to do it. Like anger and lust, Jesus stands in the tradition of the Wisdom Literature. Proverbs 12:17 and 19:5 threaten the false witness with punishment and Sirach 23:9 says “Do not accustom your mouth to oaths, nor habitually utter the name of the Holy One” and in 41:19 “Be ashamed of breaking an oath or agreement.” Like Jesus, the wisdom writers warn against hasty oaths that cannot be kept.
How does this principle of “let your yes be yes” extend to other areas of life? If we willingly break a law or policy we promised to keep, are we guilty of not “letting our yes, be yes”?
NB: While I did not cite this article, it was very helpful: Don Garlington, “Oath-Taking In The Community Of The New Age (Matthew 5:33–37)” TrinJ 16 (1995): 139–70.