Perhaps one of the most helpful illustrations I’ve both seen used and used myself in pastoral conversations is this. You take a glass and you fill it with water up to the brim and then set it on a table. Then you hit it so that water spills out. You ask those observing why the water came out and some of them will no doubt say “because you hit the glass.” More thoughtful ones will respond “because the glass was full of water.”
The point is this, I can blame other people, events or circumstances for my behaviour, whether that’s a problem with temper, course language etc but in the end, the problem is not that people and circumstances knock up against me, the problem is that those sinful habits and desires are already within me.
Jesus makes the point in response to complaints that his disciples are not strictly following the kosher laws. The accusation is that they are defiled because they are not washing their hands before eating in line with the rules and customs of Judaism. Jesus refutes this claim with a biology lesson. It’s impossible for us to contaminate our hearts by what we eat because the food passes straight through the digestive system. It doesn’t go to the heart but to the stomach and passes through.
In the immediate context, Jesus is making the point that the rituals and ceremonies people follow cannot defile them and make them unclean, conversely, nor can they make them holy. It’s helpful to remember again that rituals and ceremonies including those we get into within our own church traditions won’t change us, it’s only the work of God’s Word in our lives.
But we also need to be challenged again about the evidence of our lives. What do my words, thoughts and actions tell me about my heart? Do they show that God has been at work in my life to change me?