When Jesus tells us to call no one teacher, or call no one father, or as today to not swear; these are all examples of hyperbole. After all, the person married to my mother I did call father. The person at school I did call teacher. And that was not a sin.
What Jesus is condemning is not swearing an oath in the legal sense or swearing in the southern sense (aka cussin'), but the all too common swearing that we do to add credence when our credibility is lacking. "I swear on my mother's grave" or the famous, "I didn't do it. I swear" which is almost always used by children to cover a lie. In Jesus's time it wasn't their mothers graves but Jersualem by which people swore. In different modern languages people use different expressions. The French for example swear on the head of their mother.
In modern American speech, the more common phrase are things like "to tell the truth,..." or "to be honest,..." And we use these phrases without often realizing what they imply, that we weren't telling the truth or being honest up to that point.
What Jesus is condemning is dissembling, the half truth, the so-called white lie, all of the things that erode our credibility. As he says directly, let you yes mean yes and your no mean no. If you have to swear that what you just said was the truth, your credibility is already gone. That's the point.
As Christians we should first of all, be silent more often. Grandma was right when she admonished us, "If you don't have anything nice to say, don't say anything at all." When we are about to speak, even if it is the truth, we should ask what good is being served.
Today's gospel is not about swearing; it is about truthfulness and courage. Are there times when people demand information to which they have no right, or when to divulge it would do grave harm? Yes. But most of the time, we lie not to protect others but to protect ourselves, to simply avoid the uncomfortable situation.
Once more the gospel today calls us to pay more attention to what we say, to constantly be aware of the power of words. And perhaps more often choose silence.