Mark 7:15

There is nothing outside a man (such as food) which by going into him can defile him (morally or spiritually) but the thing which comes out of (the heart of) a man are what defile and dishonor him.  

Our hearts (soul) which contain the mind, will, and emotions, can get us into trouble if not controlled. When we guard our hearts, we protect ourselves from degrading influences and beliefs against God’s commandments. We preserve our peace and keep ourselves from sin by renewing our minds.