In verse 20 Jesus explained that it is what comes out of a man that defiles or makes him unclean, or unholy - Mark 7:20-23 (Amplified Version):
20 "And He said, What comes out of a man is what makes a man unclean and renders [him] unhallowed.
Jesus is saying what comes out of man is his words (out of the abundance of the heart the mouth speaks). What ever the think on the most is in our hearts and whatever is in our hearts will find its way coming out of our mouths in the form of health and good success for the body or poverty, sickness and disease. You see we are created in God's image and we were created with the God kind of creative power. Whatever God said He breathed life into what He said and it came to pass. Whatever we say breaths life into what we have said. We bring on ourselves blessings or cursings based on our words. When we stand before Jesus we will have to give an account for every word we have said. By our words we will be justified or acquitted, and condemn and sentenced.
Wow! That's a scary thought because I know I have said many things that I should not have said. Another scary thought is the possibility that my negative words have opened the door that defiles the body and allows the enemy to attack me with sickness, disease, and other difficulties in life, which My Heavenly Father did not intend for me to experience.
James 3 says the tongue is evil and full of deadly poison. He says the tongue can be controlled by no man. Read all of James 3 - man cannot control his words until he allows his heart to be changed through the Holy Spirit, which can only be changed through constant study and application of God's words. When His words dwell in our hearts and we say what He says then we will see success (enjoy life) and health in our lives. Read all of Book of James for a full understanding of how to control the tongue.
Jesus said that He only spoke what He sees the Father do - John 5:19, Jesus only does what pleases the Father John 5:30, Jesus only does what The Fathers says to do, John 12:49 and John 14:31, The Holy Spirit also only says and does what The Father tells Him John 16:13.
One final scripture I want to share is Colossians 3:17 (Amplified Version)
17 "And whatever you do [no matter what it is] in word or deed, do everything in the name of the Lord Jesus and in [dependence upon] His Person, giving praise to God the Father through Him."