That’s not what the poster was saying. The poster was saying that one who knows the Catholic Church to be Christ’s one Church and that she holds the fullness of truth, but nonetheless rejects her, commits a mortal sin, which continued unrepentant, would lead to death. Whereas, one who does not know and understand this is not living in mortal sin by being, say, Methodist. This has to do with the fact that in Catholic understanding, mortal sin requires not only grave matter but full knowledge and complete consent. The larger concept is that, to whom much is given, much is required. Hope this helps.