[quote="DavidFilmer, post:5, topic:274771"]
There's no such thing as an action that is always a mortal sin. People come here frequently and ask, is such-and-such a mortal sin? The answer is ALWAYS the same, no matter what the question was. The answer is always, "maybe."
In the case of leaving the Church, a slightly better answer might be, "probably not."
The act itself is only one of three requirements for the act to be mortally sinful. Another requirement is full knowledge of the sinful nature of the act.
Leaving the Church would only be mortally sinful if the person had full knowledge that the Church is the one true Church established by Jesus Christ and that all Christians ought to be in union with Her. If a person knows this but leaves anyway, it might be a mortal sin (there's still free will to consider - the third condition).
I think it's very unlikely that a person who leaves the Church knows this. A person of good will who knows that the Church is the one true Church will not leave Her - that would be irrational. People leave because they become convinced that the Church is NOT the one true Church.
It's possible that some mortal sin brought them to this belief (maybe a sin of omission by not investigating their reasons sufficiently). But the act of leaving without knowledge of the nature of the act cannot be mortally sinful.
Many left because they like their ABC, don't like to confess sins to a priest, don't like this, that, or another of her Dogma/Doctrines. Some never got Confirmed. Many went to Catholic Schools. So, where/how did they not learn about the Willful disobedience their lives demonstrate ? Again, as scripture, tradition, our Church teaches .... we are either IN CHRIST, our we are apart from him in disobedience. I might add many I know, who have left, reject Transubstantiation !!
Barnabus, Justin Martyr, Athanasius, St. Augustine, etc ...... would not be so excusatory to them as you. Lets not be the Cafeteria Catholic's enablers. We all must persevere in 'well doing' til our final day.