What does Matthew 15:18 mean?

Jesus did not require His disciples to perform the Pharisees' traditional, ritual handwashing before eating. The Pharisees had elevated this tradition to the level of doctrine. They thought it was necessary to avoid even accidentally putting a speck of a food declared "unclean" by the Law into one's mouth (Matthew 15:1–2).

When Christ dismissed this idea, He flatly stated that nothing going into a person's mouth defiles. The physical substance itself merely travels through the body and then exits. It's a physical process with no ability to travel to the soul and make it sinful. Even foods restricted for Israelites were just foods—they were not, in and of themselves, sinful. Jesus never violated those dietary restrictions (Matthew 5:17–19), nor did He teach people to do so. Rather, He insists that those rules do not exist because pork or other foods are evil in themselves.

Instead, sin and defilement already live in human souls. That defilement is revealed by the words a person says. Words show what is inside a person's heart. When we speak, we uncover the fact that we are defiled already. When a person eats something they know is forbidden, they uncover their uncleanness before God. The one who accidentally eats a tiny fleck of something, in ignorance, is not sinning.

Matthew quoted Jesus saying something similar during another confrontation with the Pharisees (Matthew 12:34). What fills the heart overflows and comes out in a person's words. It's not the eyes, but words that are the window to a person's sinful soul. If someone talks long enough, he will eventually show what's in his heart. Jesus lists some of the things in our hearts that defile us in the following verse.