The Jesus called the crowd again and said to them, 'Listen to me, all of you, and understand: there is nothing outside a person that by going in can defile, but the things that come out are what defile.'
When he had left the crowd and entered the house, his disciples asked him about the parable. He said to them, 'Then do you also not see that whatever goes into a person from outside cannot defile, since it enters, not the heart but the stomach, and goes out into the sewer?' (Thus he declared all foods clean). And he said, 'It is what comes out of a person that defiles. For it is from within, from the human heart, that evil intentions come: fornication, theft , murder, adultery, avarice, wickedness, deceit, licentiousness, envy, slander, pride, folly. All these evil things come from within, and they defile a person.'
(Church in Wales Lectionary, New Revised Standard Version)
Mark is writing his Gospel for Romans, who may not understand Jewish customs. Here he deals with the food laws (kashrut). Most of these are to be found in Leviticus and Exodus.
Food that is allowed (kosher), includes:
Food not allowed (trefah) includes:
Jesus abolishes these rules in one sweep by declaring that the only things that defile us are things that come from within us. The list he gives is quite shocking, for it includes things we not think of as sinful: envy, slander, pride, and folly.
teach us that sin does not mean just
murder, or theft, or adultery.
Sin comes from wrong desires,
from coveting what we don't possess,
from being selfish,
from being envious,
and from pride.
You might like to look up this site about food laws in the Bible: