In the context the Pharisees are questioning Him about not following the Law about eating incorrectly. Here Christ states unequivocally that it is not what goes in but what comes out that is the problem. Think that through. The rules were externals. Things that the leaders added to control behavior, specifically, what and how food was consumed. But that is not the problem. The problem is a heart condition. The people did not need rules to follow, they needed to be remade, reconstituted, reborn. No rule, or law was going to do that for them.
In fact the rules would give a false impression that they were actually pleasing God by following them. Totally missing the real issue. That was such an issue for Christ, that He, again this seems to be unlike His usual behavior, calls the crowd to them to set the record straight that this is not going to work.
We need Him. His work. His life. That is all. We have to focus on knowing Him, embracing fully the life He has provided through His life, death, resurrection, and ascension. If we add to that, we are clouding the issue.