An unpardonable sin entailing a total loss of grace WordNet 3.0 & Open English WordNet

A mortal sin, in Christian theology, is a gravely sinful act which can lead to damnation if a person does not repent of the sin before death. Wikipedia

Sinful act which can lead to damnation if a person does not repent of the sin before death Wikidata

Any of the seven deadly sins: anger, greed/avarice, envy, gluttony, lust, pride, and sloth. Wiktionary

Any of the seven deadly sins. Wiktionary (translation)

A sin which causes the loss of God's grace, leading to eternal damnation, unless the sinner repents, which includes their intent to confess and atone; such a sin is defined as a transgression against one of the Ten Commandments with the full knowledge and deliberate consent of the sinner. Wiktionary

Grave sin. Wiktionary (translation)