That's the difference in hearing with the ear and hearing with the heart.
Those "atheists and heretics" have heard with the ears but have already made the decision they don't want Christ.
They cannot hear from the heart just as the Pharisee's could not hear, they have already made that choice.
@atpollard
Paul said, "it is with the heart that man believes unto righteousness..."
Man can hear the Gospel all the day long, but if his heart is set in his own lusts he cannot hear from the heart to be saved.
The hearing of the Gospel is of none effect to him.