The Bible describes a hard-hearted person in Matthew 13:15, This is Jesus speaking, “For this people’s heart has become calloused; they hardly hear with their ears, and they have closed their eyes."

I have just an observation, you can decide it's not true, but to me it has been. The longer we are Christians, the more certain we become, the more we nurture anger and think it righteous, the higher our risk of becoming hard hearted. And throughout the Bible Gods people have often had the hardest hearts.