He was like many today who acknowledge Jesus as the Son of God. But they refuse to submit to Him as Lord. They wish to come to God on their own terms. The Bible is clear, there is no approach to God except through that which Christ has earned through his death on the cross. In order to change your focus to an inward focus, sin seeks to harden the heart.
Sin makes a man's heart stubborn.
It is simple, through deception. To make a heart stubborn, give the heart a delusion that distorts the reality. The heart is deluded by developing a hard shell around it that makes it impenetrable to outside forces. All a harden heart knows is what it feels, thinks, wants and what its needs are. All of this is distorted because its focus is limited to self.