Jesus addresses the belief that the scribes added to their traditions that they must wash their hands before they eat bread.
I read in a commentary this tradition comes from the belief that washing your hands was actually a way to wash any demons on you off of your hands, so that the demons didn't enter them through their food when they ate.
Jesus obliterates this superstition by telling them what enters the mouth to be eaten gets fed to the stomach and is eliminated.
It's not what goes into the mouth, but it is what comes out of the mouth. Because what comes out of the mouth originates from the heart and those are the things that can defile a man.
Things from the heart: evil thoughts, murders, adulteries, fornications , thefts, false witnesses, blasphemies:
To eat with unwashed hands does not defile a man.
The Pharisees are vainly worshipping with their words not with their hearts and teaching men's commandments as doctrine.
My heart. That is where all the defilement is going to come from. That is where we open ourselves up to defilement. When we take the struggles in our heart and brig them to fruition. I've read guard your heart with all diligence from from it flows the springs of life. The world's philosophy today is definitely follow your heart, but that teaching does not line up to what the Bible says. The heart is deceiving... You have to guard it, and also have The Lord set a guard over our mouth.
That is where we open the door up to struggle. Not when we eat with unwashed hands. That's the message Jesus wants to get across.