Jesus told the Parable of the Sower (Matthew 13:1-8), but the crowds did not understand what He meant.  His disciples asked Jesus privately (Matthew 13:10), and He said that the parable was really intended for them, not the crowds (Matthew 13:11-13).  Then Jesus quoted almost directly from Isaiah 6:9-10 – initially a prophecy explaining why God sent His people into exile, over 500 years earlier, when they had refused to listen to Him.  Most of Israel’s and Judah’s kings, with their people, had been apostate.  They ignored prophet after prophet.  Their hearts were hard because they rejected God’s word and worshipped idols (Psalm 135:16).
 
Jesus applied the same prophecy to the crowds who followed Him but later demanded His crucifixion.  Their hearts were hard (Mark 3:4-6).  It was not a sudden event; their hard-heartedness was well practised … to the point at which they could not feel, see or hear the truth.  What was their idolatry?  They worshipped the temple, instead of the Lord; their system of religious rules, instead of thanking the Lord for His grace and mercy.  They were proud of their own righteousness, instead of humbly repenting of their wickedness.
 
There is another edge to Jesus' quotation: because the religious majority had decided to avoid the truth by rejecting Jesus, God had confirmed their hardness.  2 Thessalonians 2:10-12 says, "They perish because they refused to love the truth and so be saved. For this reason God sends them a powerful delusion so that they will believe the lie and so that all will be condemned who have not believed the truth but have delighted in wickedness."
 
It is a solemn tragedy to be hard-hearted against God, His Son and His truth.  That is especially true when the idolatry is focussed on religious rules, habits, people and places: when the outer shell of religion has no inner meaning because the Lord Jesus is not wanted.  There comes a point at which God allows people to have their way and confirms their hardness, blindness and deafness.  Even the glorious gospel means nothing to them.  Never get to that point; and warn your friends to ... "Seek the Lord while He may be found; call on Him while He is near." (Isaiah 55:6).  Even Christian religion without knowing, loving and serving Christ is meaningless. Practised rebellion against the Lord Jesus is eternally dangerous (Matthew 7:21-23).