In this opening section there are two very different ideas about how someone becomes spiritually clean. (v1-20).
The Pharisees and scribes saw cleansing as coming from ceremonial activity (v1-2). Jesus’ response is devastating in that He shows that while they engage in ceremonial activity, they fail to keep a simple commandment like honouring their father and mother (v3-6).
The problem is really a problem of the heart (v8-9) and this is why ceremonial activity cannot make people spiritually clean. It doesn’t go deep enough.
Jesus teaches that the problem with people is not anything that enters them (v11), but rather the problem is something that is already there, the sin in people’s hearts (v18-20). In our hearts lurk the seeds of all these terrible sins.

This means that the answer to sin and the way to a clean life, has to begin with the cleansing of our hearts. This happens when the power of the cross is applied by the Holy Spirit to people’s hearts at rebirth. Baptism helps illustrate this truth. Water baptism cleanses no one within, but the baptism by the Holy Spirit can wonderfully cleanse people within.

For salvation we need the cleansing power of the Holy Spirit at rebirth, but also, we need to pray for the Spirit’s work to continue in sanctifying us.