Apostasy Is Being Stubborn and Unteachable toward the Truth of God's Word

It was not the lack of, or deception concerning, the truth about YHWH that was the sin for which He condemned Israel, but for their unwillingness to learn otherwise. They were stubborn, hard-hearted/minded, stiff-necked (i.e., unable to change direction). The disciples were just as wrong about the Messiah as the Pharisees were, but the difference between them was their humility. The heretic is a heretic, therefore, not because he believes what is wrong, but because he is unwilling to change his mind when corrected.