Nothing that enters one from outside can defile that person;
but the things that come out from within are what defile.
With these words in today's gospel Jesus reoriented the focus of faith. It should be said from the outset that this one verse cannot and should not be taken out of a larger context so as to suggest that faith is a purely interior exercise.
Jesus is dealing with the particular phenomenon that he saw in his time. At his time he witnessed many who were almost exclusively focused on the externals: food, clothing, etc. in a way that did not get to the "heart" of the matter.
Jesus offers as the starting point the heart of the person. From the heart in his language will come forth not simply what we say and do (external actions) but the intention that motivates those words and action, why we do what we do.
If we look at the words of the confiteor we will see that the words are in a particular order: thoughts, words, what I have done, what I have failed to do. It starts with the interior and radiates out. While our faith must express itself in action, those actions must have the right motivations.