Malcolm Gladwell, in his famous book Talking to Strangers, wrote about the deeply confusing depths and potential evils of the human heart, although he probably would not have termed it in such a way.
The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
Keep your heart with all vigilance, for from it flow the springs of life.
It is wise to remember that no matter how pure, holy and spotless you feel or how secure you think you are, you are not above being tempted! When you love Jesus, when he is the savior of your heart and ruler of your will, you are a marked person in hell. Satan will try to cut you down to the ground and it is a wise believer who realizes this in order to be equipped for the battle.