How do we know the human heart is the source of sin?

In this passage, Jesus clarifies that external observances do not determine purity; rather, it is the condition of the heart that matters. He illustrates this by declaring that from the heart come evil thoughts, murders, adulteries, thefts, and blasphemies, indicating that true defilement originates internally. By emphasizing that even seemingly respectable individuals have hearts that can produce such evils, Jesus confronts the self-righteousness of the Pharisees and all people. This teaching reinforces the need for spiritual renewal, urging believers to seek a clean heart through Christ, as nothing external can ultimately cleanse one’s sin.