What does the Bible say about the nature of evil in humans?

According to Mark 7:14-23, Jesus illustrates that the root of human sin lies within the heart rather than in external actions or societal rules. He emphasizes that the evil thoughts and intentions that arise from the heart define a person, pointing to the need for transformation rather than mere reformation. The parable of a pig rolling in the mud highlights that simply cleaning the outside does not change the inner nature. This understanding reinforces the doctrine of total depravity, which asserts that humanity's fallen nature is the source of all sin and evil, demanding a fundamental change through God's sovereign grace.