In the Old Testament, there were many regulations as to what you could or couldn't eat. For example, if an animal had a divided hoof and chewed it's cud, it was considered clean and the Israelites could eat it. On the other hand, if something didn't meet the qualifications, it was considered unclean. So if someone ate it, they were unclean.
The Pharisees approached Jesus and his disciples in Mark 7 about their eating with defiled hands. Jesus replied by telling them that they are so focused on their traditions, that they are forgetting the commands of God. He tells them that nothing that goes into you can defile you, if is what comes out. What comes out of you is from your heart and only that can make you clean or unclean.
Mark 7:1-15, 17-23 NIV
The Pharisees and some of the teachers of the law who had come from Jerusalem gathered around Jesus and saw some of his disciples eating food with hands that were defiled, that is, unwashed. (The Pharisees and all the Jews do not eat unless they give their hands a ceremonial washing, holding to the tradition of the elders. When they come from the marketplace they do not eat unless they wash. And they observe many other traditions, such as the washing of cups, pitchers and kettles.) So the Pharisees and teachers of the law asked Jesus, "Why don't your disciples live according to the tradition of the elders instead of eating their food with defiled hands?" He replied, "Isaiah was right when he prophesied about you hypocrites; as it is written:
"'These people honor me with their lips,
but their hearts are far from me. They worship me in vain;
their teachings are merely human rules.' You have let go of the commands of God and are holding on to human traditions." And he continued, "You have a fine way of setting aside the commands of God in order to observe your own traditions! For Moses said, 'Honor your father and mother,' and, 'Anyone who curses their father or mother is to be put to death.' But you say that if anyone declares that what might have been used to help their father or mother is Corban (that is, devoted to God)— then you no longer let them do anything for their father or mother. Thus you nullify the word of God by your tradition that you have handed down. And you do many things like that." Again Jesus called the crowd to him and said, "Listen to me, everyone, and understand this. Nothing outside a person can defile them by going into them. Rather, it is what comes out of a person that defiles them." After he had left the crowd and entered the house, his disciples asked him about this parable. "Are you so dull?" he asked. "Don't you see that nothing that enters a person from the outside can defile them? For it doesn't go into their heart but into their stomach, and then out of the body." (In saying this, Jesus declared all foods clean.) He went on: "What comes out of a person is what defiles them. For it is from within, out of a person's heart, that evil thoughts come—sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. All these evils come from inside and defile a person."
In The Message translation, it quotes Jesus as saying in verses 20-23, "It's what comes out of a person that pollutes: obscenities, lusts, thefts, murders, adulteries, greed, depravity, deceptive dealings, carousing, mean looks, slander, arrogance, foolishness-all these are vomit from the heart. There is the source of your pollution."
In Luke 6:45, Jesus addresses this subject yet again. "A good man brings good things out of the good stored up in his heart, and an evil man brings evil things out of the evil stored up in his heart. For the mouth speaks what the heart is full of."
We must search our hearts and our minds to know if it is good or evil that we are storing up in our hearts. Because whatever is stored up within us WILL COME OUT! If a water bottle is full of water and you squeeze it, what comes out? Water because that is what is inside of the bottle. What is stored up inside of you? What is coming out of you? What is in your vomit? Is your heart a source of pollution?