I was always told that ANY sin could be a sin against hs. The issue is not what the person does, but their attitude. So if someone willfully does anything, and it's not about human weakness, then that would qualify. Apparently it was this type of sin that Adam committed---because he had to turn his heart completely away from his god to commit it. They teach that a person guilty of such sin has moved beyond the capacity to repent. Their heart has been destroyed, they can't repent, nor do they desire to do so, and therefore, can't be reinstated or forgiven.
In practice however, I think they focus most strongly on apostacy. Because in their eyes, we still know they have the truth, but we willfully turn away and say it is not the truth. We are in full control of our actions and have chosen evil. So we are unrepentent not because we don't think repentence is necessary, but because we are unable to repent and revive our hearts. We have experienced spiritual death.
Many people suffer depression because they fear they have inadvertantly committed the unforgivable sin. The Depression tract tells them that this fear is proof that they did not. An unrepentent sinner would not fear or even care if they had.
It's a wonderful way to keep people insecure and afraid to question. The scripture bothered me so much at one time, I actually called into a Christian radio show and asked them to explain it. I didn't understand the explanation, but in this guys eyes, Jesus would have had to be on earth for us to have committed such a sin.
I'm so glad that I've given up the idea of sin. It's a heavy load. Now there are only actions and consequences---which means there are things one should not do---but there is not a cosmic balance sheet.
NC