If I were to leave the Catholic Church and become a Protestant with full knowledge of what I a doing I would be committing heresy.
If I were to leave the Catholic Church and become an Atheist or Hindu I would be committing apostasy.
If I were to leave the Catholic Church and become a Jehovah Witness or Mormon what would I be considering they are not Christians in the sense we are as they don’t put Jesus on the same level as we do but they still believe in Jesus to some extent?
My understanding is someone who is heretical is someone who is in the faith, but espouses contrary POV. Basically any “Cafeteria Catholics” who don’t adhere to Catholic teaching and publicly deny it would be heretics. Meanwhile anyone who leaves the faith, for a Christian faith or otherwise would be an apostate.
That is unless you believe that other Christian faiths (ie: Trinitarian), aren’t necessarily a different faith, just imperfect versions of a larger Christian faith, in which case then yes it could conceivably be heresy instead.
Heresy is the rejection of Truth as defined by the Catholic Church.
Apostasy is total repudiation of the Christian faith.
Schism is refusal to submit to the pope or communion with the Catholic faithful.
So a Catholic that leaves the Church is first off a schismatic. I would guess they also are guilty of heresy, but I suspect there might be a way to schism without commiting heresy. A life long Presbyterian might hold the same heretical beliefs as the schismatic Catholic, but since they never professed the Catholic faith they would be considered a material, rather than a formal heretic.
So the real question is if a Catholic leaves the Church for a nontrinitarian sect if that is a total repudiation of Christianity? That is a tough one. LDS, JW and others hold many views that are certainly heretical, but I don’t know if that raises to the level of apostasy.
I guess the real question is if you layer enough heresies on top of one another are you a material apostate even if you still profess to hold a Christian faith? It would seem that it would be apostasy if you think of a Hindu that might claim Christ as an incarnation of Krishna. They might revere him and try to follow his example, but that does not make them Christian. For certain sects it would seem their errors are so overwhelming that they have a completely different faith simply based on the same set of writtings. In that case it would seem to be a form of apostasy though I don’t know if it would be considered a formal form.
Guess that makes sense from a Catholic perspective. I mean the Eastern Orthodox churches are in schism from Rome in large part due to their repudiation of the Pope’s authority and someone who say leaves Catholicism for an Eastern Orthodox church would be doing much the same.
With regard to the dictionary definitions: An apostate is a person who has renounced a religion or faith. Where a heretic is someone who, in the opinion of others, believes contrary to the fundamental tenets of a religion he claims to belong to. And a schism is the formal separation of a church into two churches or the secession of a group owing to doctrinal and other differences. Which may be where the difference of my opinion lay. The Catholic and Dictionary definitions don’t quite line up.
But yes per the Catholic POV’s the OP’s reading appears correct.