From Longman Dictionary of Contemporary Englishmortal sinˌmortal ˈsin noun [countable, uncountable] RRCsomething that you do that is so bad, according to the Roman Catholic Church, that your soul will be punished for ever after death unless you ask to be forgiven
Examples from the Corpus
mortal sin• Martinho would have accounted his act a mortal sin.• And is hers strong enough to marry a man who regards unpunctuality almost as a mortal sin?• She regarded gossip as a mortal sin, and encouraging it almost as bad.• Making fun of the Corps is a mortal sin.• Committing suicide was a mortal sin.• As I have said, we are not dealing with a demon but something worse, a soul steeped in mortal sin.• Suppose, suppose, she were in a state of mortal sin?• He had compounded a number of venial failings with the mortal sin of adultery. 