To this someone may reply that he regarded the promise made in church as a mere formality and never intended to keep it. Whom, then, was he trying to deceive when he made it? God? That was really very unwise. Himself? That was not very much wiser. The bride, or bridegroom. or the ‘in-laws’? That was treacherous. Most often, I think, the couple (or one of them) hoped to deceive the public. They wanted the respectability that is attached to marriage without intending to pay the price: that is, they were impostors, they cheated. If they are still contented cheats, I have nothing to say to them: who would urge the high and hard duty of chastity on people who have not yet wished to be mere honest? If they have now come to their senses and want to be honest, their promise, already made, constrains them. And this, you will see, comes under the heading of justice, not that of chastity. If people do not believe in permanent marriage, it is perhaps better that they should live together unmarried than that they should make vows they do not mean to keep. It is true that by living together without marriage they will be guilty (in Christian eyes) of fornication. But one fault is not mended by adding another: unchastity is not improved by adding perjury.
The central point is this: Jesus' entire approach in the Sermon on the Mount is not only ethical but messianic—i.e., christological and eschatological. Jesus is not an ordinary prophet who says, "Thus says the Lord!" Rather, he speaks in the first person and claims that his teaching fulfills the Old Testament; that he determines who enters the messianic kingdom; that as the Divine Judge he pronounces banishment; that the true heirs of the kingdom would be persecuted for their allegiance to him; and that he alone fully knows the will of his Father.