"He, that being often reproved hardeneth his neck, shall suddenly be destroyed, and that without remedy" (Proverbs 29:1).
The Hebrew word for reproved in this verse refers to corrective teaching. And the words for without remedy mean "without a cure, without any possibility of deliverance." This verse first tells us that hardness of heart comes as a result of rejecting repeated warnings and pushing aside all wooing of the truth. Second, it tells us that over time such hardness becomes impossible to cure. So, who are the people who most often hear these warnings? They are supposedly Christians, those who sit in God's house each week listening to sermons of reproof.
You may ask, "What exactly is a hard heart?" It is one that is determined to resist obeying God's Word, impossible to stir, and immune to the convictions and warnings of the Holy Spirit.
The tragic truth is that in spite of hearing fiery messages sent from heaven, multitudes of Christians do not practice what they hear. They refuse to allow God's entry into certain areas of their lives and as they continue to hear without heeding, hardness begins to set in.
In contrast, there are many sinners whose hardness of heart has been cured. At first they cursed Christ and shook an angry fist in God's face. But when they heard the gospel and felt the pure, loving reproof of the Holy Spirit, their hearts melted. They repented and turned to Jesus.
The life of Madalyn Murray O'Hair's son illustrates this. He had been reared in probably the most atheistic household in America and he later worked for his mother, crusading against God and religion. But when he heard the gospel, he got gloriously saved and became a minister, preaching Christ instead of cursing Him. This man's hardness was curable also—because he had not sat under sermons of reproof and continually rejected them.
In my experience, the hardest hearts—the incurable kind—have always been found within earshot of Holy Spirit-anointed preaching. Such hardness does not exist in cold, dead, formal churches where the gospel has been corrupted for generations. No, it is always found where a pure word is preached from the pulpit and rejected in the pews.