First, a hard heart keeps us from perceiving spiritual truths. This is why everyone in a church service hears exactly the same message yet, some will receive while others won't. It's not the Word that is the variable, but rather the condition of the hearts.
Second, a hardened heart stops us from understanding spiritual truth. When a person doesn't understand God's Word, Satan finds no resistance when he comes to steal it away.
Third, a hard heart keeps us from remembering. This isn't to say that we can't recall facts or scriptures. In Mark 8:19-20, the disciples remembered the facts of the two miraculous feedings, but they had forgotten any spiritual lessons they might have learned.
Some people can quote scripture or remember what the sermon was about, but they can't perceive the spiritual life in the message or retain what they did perceived.
A hardned heart blinds us to any spiritual perception and keeps us thinking only in the natural realm. Small faith and a hardened heart are the same. Hebrews 3:12-13 parallels an evil heart of unbelief to a hardened heart. It takes great faith to keep our heart sensitive to God.
