What does Matthew 5:33 mean?

Jesus continues to give one example after another, comparing what His hearers have been taught by Israel's religious leaders to God's true intent for the hearts of His people. His pattern is not to reject the words of the religious leaders, but to show how their interpretations fall short (Matthew 5:20). The full meaning comes as Jesus follows each statement by saying, "I say…" So far, He has given a deeper understanding of anger (Matthew 5:21–22), adultery (Matthew 5:27–28), and divorce (Matthew 5:31–32).

Now Jesus turns to the issue of swearing an oath. This is not about bad language. Nor is it a reference to serious, formal promises, such as those seen in wedding vows or a courtroom. Rather, Jesus is speaking of the use of God's name as a token to seal a promise. He's also speaking of the practice of adding some qualifier to our words to declare honesty—such as "cross my heart," or "I swear on my mother".

Numbers 30:2 describes it like this, "If a man vows a vow to the LORD, or swears an oath to bind himself by a pledge, he shall not break his word. He shall do according to all that proceeds out of his mouth." Under that understanding, someone might say, "I swear to the Lord that I will pay you back this money." Among Israelites, this have been considered contractually binding. It was also seen as dangerous: breaking an oath to the Lord was understood to bring severe consequences.

In practice, however, this concept became yet another loophole subject to abuse. In some cases, oath-breakers might argue that if God had meant the oath to be kept, He would have ensured it, therefore the oath was never binding. Swearing by other things and places would have provided even more wiggle room to the oath-taker. Obviously, this creates a broad opportunity for premeditated deceit. Israel's religious leaders may have made the problem worse by debating which oaths were binding and which were not.

Jesus sets a different, more challenging standard for His disciples in the following verse.