The unpardonable sin is the sin of blasphemy against the Holy Spirit. Showing irreverence toward something sacred or toward God is considered blasphemous in the Bible.
Some people have difficulty with the idea of God not forgiving all sins. In Christianity, the doctrine holds that if a person confesses his sins, God absolves him and forgives all his sins. This seems to be a contradiction to passages alluding to the unpardonable, or unforgivable, sin. The only way a man could commit an unforgivable sin is to reject Christ's offer of salvation.
Acceptance of salvation equals belief. Rejection of salvation equals disbelief. The man remains in a state of unbelief and unrighteousness and is unforgivable. According to church teachings, if he never accepts salvation, Christ does not force it upon him.