What does the Bible say about a believing heart?

The Scriptures illustrate that a believing heart is essential for salvation and understanding of God's truth. It is described as tender, thankful, and established by faith, differing fundamentally from a hardened heart, which is impervious and unbelieving. In Hebrews 3, we are urged not to harden our hearts but to be responsive to God's voice, highlighting the necessity of grace in developing a believing heart. This transformation is not achieved through human effort but through the supernatural work of God's grace.