What happened to their hearts? They had been hearing the good news from Jesus Christ time after time after time to the point where their heart responds to the Lord and they develop a condition. Do you realize that a person can develop callousness? That they can reach the point where they cannot be sensitive to the things of the Lord anymore? Everybody's heart is being conditioned by hearing the word. Either being receptive to the word, or developing a resistance and a callousness towards the hearing of the word.
