In the context of first century Judaism, this could easily be seen as the most revolutionary of the sayings of Jesus. He has been having an argument with the religious leaders about the extra burden they have imposed on the Jews, all of whom were already hugely weighed down by poverty, Roman occupation, sickness and confusion. Jesus has shown how elaborate hand washings are irrelevant; rigid adherence to traditional law can become disobedience to the Law of God and here He declares that nothing which goes into a person from the outside can defile them! We can glance through Leviticus chapter eleven and discover a long list which are declared unclean by God, but Jesus with supreme authority wipes out a whole list of dietary and ritual Laws over which the Jewish people had died rather than disobey.
Jesus goes on to state that it is what is within a person which has the potential to be unclean and we can agree the truth of this in our own actions stemming from envy, guilt or shame and the evil people have done to us out of their damaged hearts. Firstly, He mentions evil designs because everything which is an evil act comes from a decision in the heart to commit that sin. From there He goes on to cite a whole list of sins, one at least of which we would recognise from our own lives and acts. It is always the same in the New Testament, I think, that the 'big' sinful acts- sexual immorality, theft, murder are mixed in with the everyday acts, such as slander, pride and envy: in God's eyes, a sin is a sin and it all needs rooting out, He wants us to be perfect!
Hopefully, one of our responses to this passage is to spend time in self-reflection. We all think of ourselves as 'good' people and it can be ground-shaking when we are shown to be full of evil, rather than sweetness and light. I'm not saying that we should be in a continual state of guilt and beating ourselves up about who we are, but the Christian faith can give us the strength to face up to who we are and to trust more fully on God's mercy and grace.