20 He went on: “What comes out of a person is what defiles them.
21 For it is from within, out of a person’s heart, that evil thoughts come—sexual immorality, theft, murder,
22 adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly.
23 All these evils come from inside and defile a person.”
Basically what is being said here is that all of our sins come from the heart of man. When we remain without God's Guidance, help, spirit we remain a sinful being.
We can be extreme discipline and work become one of the "good" people. People who want to do good who help others and try to live a decent life but we retain our sinful hearts and are sure to eventually fail. It can't be helped we can not be Godly using our sinful heart. We will stumble at beast falling into the vices of the flesh. Be it greed, jealousy, vice, whatever.. We are more susceptible to fall .
In order to serve God, truly and faithfully, we need an entirely new heart. We need God's spirit to cleanse us of our fallen flesh and rejuvenate us with His heart and Spirit.
We will still be tempted, we might still stumble and we might still fall but that will be when we deny ourselves of God's Spirit and Heart.
*remember these are my thoughts on this verse and what I took away from my Bible study.. feel free to add yours to the discussion.