How do we know that spiritual defilement comes from the heart?

In Mark 7:21-23, Jesus explicitly states that 'from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders,' and other sinful behaviors. This teaching underscores that the problem of sin is not external but internal. Natural men often blame external circumstances or influences for sinful behavior, but Scripture reveals that our inherent sinful nature is what truly defiles us. The heart, therefore, is the wellspring of both good and evil, and until it is transformed by God's grace, the sinful inclinations will continue to arise from within. This insight highlights the need for regeneration—changing of the heart—rather than mere behavioral modifications.