How deep does one's understanding have to be, in order for a sin to be mortal?

I’ve often wondered about this, when meditating on sin. In order to be culpable of a mortal sin, how deep/nuanced does one’s understanding of the sin’s gravity have to be? Is it enough to know: “This is a really bad thing to do,” or does one have to actually realize: “This is a really bad thing to do. If I do this, I’ll throw away sanctifying grace and be unfit for Heaven.” The latter seems to make the choice to commit mortal sin so much more deliberate, but so many mortal sins are forbidden by natural law, that the former also makes sense. Which is the correct understanding?


A non-Catholic is quite capable of committing a mortal sin even without knowing anything about the term ‘mortal’ sin. Natural law can certainly tell us that stealing or taking human life is a very grave evil. Certainly, the more one knows about the seriousness of an act of mortal sin, the more culpable one may be; so both, in a sense, are true.

Fr. Vincent Serpa, O.P

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