What does the Bible say about the hardness of the human heart?

The Bible reveals the desperation of human hardness and unbelief, as demonstrated in John 7:1-13, where even Christ's own brethren did not believe in Him. This illustrates the doctrine of man's absolute need for preventing and converting grace, showing that spiritual privileges do not result in genuine faith. Observing Christ's miracles, hearing His teachings, and living near Him were insufficient to move them to faith. It emphasizes that without the effectual work of the Holy Spirit, even the most privileged can remain unbelieving.