What does the Bible say about the unconverted heart?

In Luke 7:31-35, Jesus likens unconverted men to children who are never satisfied, constantly finding fault with God's messengers. This characterizes the hearts of those who are unconverted; they harbor a deep-seated animosity towards God's truth and will consistently reject what God offers. Even in the presence of John the Baptist and Jesus, who represented two extremes of godly living, the Jews remained obstinate, proving that their objections were mere veils for their hatred of divine truth. This concept is crucial for understanding human nature in relation to God's sovereign grace, as it highlights the extent of rebellion and resistance found in the unregenerate heart.