The text of today continues the discussion of the earlier text, which was read yesterday (7,1-13). If the earlier part was a response to Jewish teachers, this part is addressed to the crowds. Jesus asserts that nothing from outside has the power to make one unclean. Instead of being concerned with externals, Jesus challenges those who listen to him to focus on the internal, since uncleanness comes from within. Mark presents this teaching of Jesus as a parable and so there is a need to explain it. In his explanation to the disciples, Jesus makes clear that what goes into a person from outside enters the stomach and not the heart and so cannot defile. It is what comes from within, that is from the heart that defiles and makes unclean.
Sin comes from within. While external circumstances do have an effect on us and influence us, we cannot put the blame for our actions on these. The actions that we perform are ours and we must accept responsibility for them.