What does Matthew 23:18 mean?

Jesus is revealing the selfish motives and foolishness of the scribes and Pharisees in creating categories of oaths. He has already said that they distinguish between swearing by the temple and swearing by the gold that is in the temple. According to these religious leaders, making a vow on the gold creates a binding agreement, but doing so on the temple alone means nothing (Matthew 23:16–17).

The scribes and Pharisees have made a similar rule about swearing by the altar versus by the sacrifice given on the altar. As they would have it, swearing by the gift is binding, but swearing by the altar itself means nothing. This complicated system seems to have grown from a restriction on swearing on or by God Himself. Religious Jews at this time likely did not even say the name for God. Instead, they began to swear by things related to God when making oaths.

While adding extra "certification" to a promise is bad enough (Matthew 5:33–37), the suggestion that swearing by one thing as binding and another as non-binding reveals the Pharisees' lack of understanding about the nature of God and what it means to swear an oath. In this way, they are "blind guides" and "blind fools." They are leading those they teach away from what is wise and true (Matthew 23:13–15).