In ancient times, oaths were far more common, and in Jesus’ culture one made a serious oath by swearing to God; now we hear it as a flippant, throw-away phrase. “I swear to God…” However, in that age it was believed—and culturally enforced—that if you swore to God you would be and were held accountable and you had to keep your word. So people got clever: they would swear by heaven, by the earth, by Jerusalem, or some other holy thing so that in the moment of making the oath, the other person would assume it was equivalent to swearing to God. When the oath-maker became an oath-breaker, he had a convenient loophole: only oaths made to God were to be absolutely kept. Jesus closes this loophole by pointing out that all of these things—even one’s own head—belong to God. If you swear an oath, you are swearing to God, period, so you’d better mean it.
Rather than swearing as a means by which to assure someone else that you will do what you say, Jesus tells us to “Just do it.” Let your yes or no mean yes or no, and have no other plans to act contrary to what you say. Let your word and your heart be in one accord, because otherwise you are not following the Truth—Jesus—but rather Satan, who is “the Father of Lies.” (John 8:44)
In short, these last few days of Jesus’ teaching on how to truly keep the commandments have been about being a person of integrity, of our heart and actions being of one accord, rather than in discord: integrity, rather than disintegration. To live with your heart desiring one thing and your actions doing another, or to live in opposition to the Truth for whom we were created, is to exist in a sort of living death; we cannot be fully alive when there isn’t a communion of self, and likewise between that self and God. In all these cases—murder and hate, adultery and lust, marriage and divorce, false oaths and vows—Truth and Love are the keys to unlocking the chains of sin that can bind us in these and many other areas. If we love someone in our hearts, then we certainly won’t murder them in our actions. If we are faithful to those we love in our hearts, then we will be faithful to them in our actions. If we rely on the grace God gives us to love our spouses as Christ and the Church love each other (Ephesians 5:21-33) then divorce will not occur. If we simply intend to do as we say we will do, then we needn’t make vows, or fear breaking them.
How will the world know that we are disciples of Jesus? Not by our knowledge of doctrine or Scripture, not by our belief in Him, but “this is how all will know that you are my disciples, if you have love for one another.” (John 13:35) The Law finds its fulfillment in love, the love which Jesus not only teaches, but equips us by grace to live out. By His grace the Law shapes our hearts and trains them to love as He loves, and thus come to live as He lives: forever, and united in the love of the Most Holy Trinity.