What does Matthew 5:36 mean?

At the time Jesus gave this teaching, oaths were being used—and abused—in many situations. The oaths in question were not formal agreements, such as contracts, courtrooms, or a marriage. What Jesus condemns here are the casual oaths people make to add weight to a promise. In modern times, people might say something like "I swear, I'm telling the truth." That kind of oath misunderstands God's intent, which is for honest communication (Matthew 5:37). It's also subject to deception.

Some people in Jesus' era may have avoided swearing oaths by the Lord, since the consequences of not following through on those oaths for any reason were understood to be harsh (Numbers 30:2). However, the religious leaders apparently allowed for swearing oaths on other things, as a way of cementing agreements. So, someone might say, "I swear by heaven and earth that I will do this thing."

Jesus says this is wrong. Swearing by anything that belongs to God or is under His control comes too close to swearing by the Lord. He has described heaven as God's throne, earth as His footstool, and Jerusalem as His city. Swearing an oath by what He owns is just a substitute for swearing by the Lord Himself. It is a dangerous practice.

Now Jesus takes this prohibition even further. He tells them not to take an oath even by their own heads. This might be the equivalent to saying something like, "I swear by my life that I will do this." Or, "I swear by my eyes that I will do that." Jesus, though, points out that humans don't even have control over the color of the hair on their heads. Our heads belong to God, as well.

He drives the point home in the following verse: Christians ought to be known for honesty, such that their simple word is good enough (Matthew 5:37).