In the bible, stiff-neck refers to inflexible obstinacy. It doesn't necessarily point to any specific sin. It simply describes someone who is unwilling to change. It is a stubborn attitude resistant to transformation.
At the root of the biblical message is transformation. Change. Paul was right to indicate that true transformation begins in the mind: "Be transformed by the renewing of your mind" (Romans 12: 2). Behavior modification is one thing. Mind modification is another.
Actually, it's not about modifying the mind, but renewing it. Behold, he is a new creation! We're talking about the kind of change that is radical. Not change but exchange.