Truth is, humans have a heart of darkness.  Left to our own devices, or trusting in anything (idols) other than love from God will trap us there.   Jesus referred to this one time while he was in Jerusalem:

…from within, out of men’s hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly.  All these evils come from inside and make a man ‘unclean. Mark 7:21-23 (NIV)

Too many times we read these words and begin to pass judgment on a host of people who we just KNOW are living in darkness because they are not like us.  We are too good to live in darkness – and we must have missed it when Jesus chastised the Pharisees and teachers of the law just moments earlier.

…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 but rules taught by men.”
You have let go of the commands of God and are holding on to the traditions of men.  Mark 7:6-8 (NIV)

The Pharisees Jesus was talking to were considered the conservatives of Judaism. They held strictly to the Torah and the Talmud and were very outwardly moral. But because they trusted in the traditions of men (meaning adding to the law) they were far from him, they had hearts of darkness.