“Theological Time-Out”
Jesus is referring to the expansion of the law; those traditions for oath taking: how you hold your hand, what to do to prove you're telling the truth. Jesus decides to throw out those expansions of the law.
Now comes the grand simplification of the ninth commandment: "Let what you say be simply 'Yes' or 'No'; anything more than this comes from evil.
Look what Jesus has done: he has brought us into contact with the grand positive of this commandment. This commandment is about our relationship with reality. Our relationship with truth.
It is about a truthful relationship we have with ourselves, it's about a truthful relationship with God, and truthful relationships with our neighbors around us. Jesus spoke about oaths in order to point out that they were not the main problem—integrity was. Oaths are no substitute for personal integrity.
A liar’s vow expresses a worthless promise. But when a person of integrity says yes or no, that person’s simple word can be trusted.
Jesus is calling us to make integrity our standard.
The Pharisees, in contrast to Jesus’ insistence upon straightforward truthfulness, had expanded oaths to matters of technicalities, much like our own American legal system that often twists and turns the normal meaning of words. Legal technicalities can devolve to the point of destroying the justice they are supposed to serve.Truth loses itself in wordiness.
The pharisaical system of oaths was expanded to cover all kinds of promises made to one another. “Levels of truth” emerged from the varieties of oaths. were considered more binding than oaths on the earth.
Like children crossing their fingers behind their backs, the Pharisees camouflaged reality in elaborate technicalities.
The point Jesus is making is that your yes should be yes and your, no, no; and an oath should not be necessary in order to verify the veracity or accuracy of your statements.