To those who reject the word of God, the judgment of God and the consequence of a godless life remains upon them. That is why in Mark 4:12, Jesus paraphrases something from Isaiah 6:9, on why He spoke in parables. “so that they may indeed see but not perceive, and may indeed hear but not understand, lest they should turn and be forgiven.”
The idea behind the verse is because of Israel’s stubbornness and unwillingness to hear the word of God. The consequence was an inability to see and understand. As a result, instead of turning and receiving forgiveness and salvation, judgment remained upon them.
In Romans 1:19-25, a similar idea is there about those who reject God, “what can be known about God is plain to them, because God has shown it to them. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking… Therefore God gave them up… because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever!”
This is perhaps one of the harder truths of Scripture to accept, that after a person hears and finds out about God, they rejects Him. Instead of receiving forgiveness and salvation, the unbeliever remains “futile in his thinking… serving the created things instead of their creator.” And so Jesus in Mark 4 warns us, "he who has ears let him hear." (Mark 4:9)
Jesus warned us another way in John 3:17-18, “For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God.”
The wonderful good news of the gospel is that Jesus came. But it only becomes good news when a person believes and puts his trust in Him. When a person rejects Christ or ignore His words, though he or she may have physical ears to hear, their hearts remain untouched. And so Jesus pleads and warns those who have a hardened heart , “He who has ears let him hear.” (Mark 4:9)