mortal sin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A sin, such as murder or blasphemy, that is so heinous it deprives the soul of sanctifying grace and causes damnation if unforgiven at the time of death.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Roman Catholic theology a mortal sin is a grave matter, such as murder or blasphemy, and is committed with the full knowledge and deliberate consent of the sinner. The committing of such a heinous sin causes a complete loss of grace and leads to eternal damnation unless absolved.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an unpardonable sin entailing a total loss of grace