What Jewish practice caused Jesus to condemn the swearing of oaths?
OATHS had their proper place according to the Mosaic Law. However, their use became so common in the everyday life of Jesus’ contemporaries that there was the tendency to reinforce practically any utterance by swearing to it. This frivolous practice was intended to add credence to one’s statements; yet, Jesus twice denounced it. Instead, he taught: “Just let your word ‘Yes’ mean yes, your ‘No,’ no.”
According to the Theological Dictionary of the New Testament, that “the inclination of the Jewish people to confirm all possible statements by an oath” was very strong may be discerned from passages of the Talmud, which painstakingly specifies which oaths were to be considered binding and which were not.
Jesus was not alone in condemning these abuses. For example, the Jewish historian Flavius Josephus said of a Jewish sect: “Swearing they avoid, regarding it as worse than perjury, for they say that one who is not believed without an appeal to God stands condemned already.” The Jewish apocryphal writing known as the Wisdom of Sirach, or Ecclesiasticus, (23:11) likewise states: “A man given to swearing is lawless to the core.” Jesus, then, condemned the practice of taking oaths lightly. If we speak the truth at all times, we should not have to take an oath to make our words more believable.