Someone leaves the Catholic Church because they think it does not hold the fullness of truth and they seek the find the truth.
OK – so, stop for a second and think about what you’ve just written…
Is it true that “the Catholic Church does not hold the fullness of truth”?
If so, then the person has judged correctly, and there’s clearly no sin.
If not – if the Church
does hold the fullness of the truth, then the person has judged incorrectly. Moreover, in reaching this false conclusion, he commits sin (i.e., he leaves the Church). The question in this case becomes “is the person responsible for his false reasoning which led him into sin?”
So why would anyone trust the Catholic view that it is a sin to leave the Church when they no longer believe in the Church
If the Church truly
does have “the fullness of the truth”, then it’s a grave sin to leave. The only question is whether this rises to “mortal sin”, or is merely “venial sin”. A person should trust the logic of the proposition, then, and not the question of belief.
(On the other hand, what you’re talking about, of course, is ‘conscience.’ And yes – if, in his conscience, a person does not believe that the Church has the fullness of the truth, then he’s obligated to follow his conscience. However, a person is also
responsible for the proper formation of his conscience, such that it tells him the truth. So, yes … in this case, he must leave the Church;
but, he’s morally responsible for that decision! So, his decision to leave
might be imputable to him as mortally sinful!)