How do we know that we must rend our hearts?

In Joel 2:12-14, God commands His people to 'rend your heart and not your garments,' indicating that true repentance and turning to Him requires an internal transformation rather than mere ritualistic actions. This is affirmed in the New Testament, where Paul clarifies that true circumcision is of the heart (Romans 2:29). This reinforces the belief that we cannot change our hearts by ourselves and must depend on God's grace to effect this change within us. It illustrates our need to cry out for mercy, acknowledging that He must be the one to rend our hearts and give us a new heart that seeks after Him.