“Woe to you, teachers of the law and Pharisees, you hypocrites! You clean the outside of the cup and dish, but inside they are full of greed and self-indulgence. Blind Pharisee! First clean the inside of the cup and dish, and then the outside also will be clean. Woe to you, teachers of the law and Pharisees, you hypocrites! You are like whitewashed tombs, which look beautiful on the outside, but on the inside are full of dead men’s bones and everything unclean. In the same way, on the outside you appear to people as righteous but on the inside you are full of hypocrisy and wickedness” (Matthew 23:25-28).
“You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks” (Matthew 12:34).
Quoting Isaiah, Jesus said,
“‘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’” (Matthew 15:8-9).
Explaining to the disciples, He said,
“Out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander. These are what make a man ‘unclean’…” (Matthew 15:19-20).
This is the impure heart.
To have a pure heart means to have a heart that is committed to living a life that is totally pleasing to God, because “… the Lord searches every heart and understands every motive behind the thoughts” (1 Chronicles 28:9). That is why David’s prayer was:
Search me, O God, and know my heart;
test me and know my anxious thoughts.
See if there is any offensive way in me,
and lead me in the way everlasting (Psalm 139:23-24).
Blessed are the pure in heart, for they will see God. (Matthew 5:8)
(From this great article)