Swearing played a significant role in Jesus’ world, and that was true not only in Israel but throughout antiquity in general. Still, many ancient philosophers and wise persons said that swearing was unworthy, and there were naturally such voices also in Israel (cf., e.g., Sir 23:9-11), where, above all, there was a profound unwillingness to speak the name of God—and every oath brought God into the event. Therefore here, as in other cases, the word “God” was avoided. Instead, people swore “by heaven” or “by earth” or “by the temple.” . . .
Jesus rejects all these oath formulae, together with swearing itself, in the sharpest terms. For him, swearing blasphemes the name of God. It instrumentalises God for earthly concerns when it would be fully adequate simply to speak the truth. Basically, this is about the Second Commandment: human beings must not misuse the holy name of God for their own purposes (Exod 20:7; Deut 5:11).
But something else must also have been important to Jesus where the prohibition of swearing is concerned: swearing presupposes a society in which one must constantly reckon with untruths, lies, and deliberate false statements. The individual is thus forced to create forms of speech for important cases, forms that signal that one is absolutely not lying. The state, in turn, has to institutionalize such forms and punish perjury. But the reign of God must proceed on different assumptions: no longer that others might lie, but that they speak the truth.
What is presupposed here is not only a different image of society; it is also a different image of the human being. When living in the truth of the reign of God, a person can be utterly simple, straightforward, and truthful—and in that way also utterly sincere.

Gerhard Lohfink, adapted from The Most Important Words of Jesus

Gerhard Lohfink was professor of New Testament exegesis at the University of Tübingen. His many books include No Irrelevant Jesus and The Forty Parables of Jesus.