Stubborn: Do you have Church docuмents that support your interpretation of Mystici Corporus Christi....that a person that commits the sin of heresy, schism, or apostasy "only" needs to go to confession?
Do you have Church docuмentation that supports "once a Catholic, always a Catholic"?
These are 2 distinct questions. When you combine them together, you get wrong answers.
1. For simple heresy, yes, confession suffices. For major heresy, apostasy, schism - it doesn't. You have to do more than just go to confession.
2. "Once a Catholic, always a Catholic" -- this has to be looked at in multiple ways.
a. Can one get re-baptized, or baptized twice? No. Therefore, in regards to Church membership -- once a member, always a member.
b. Can one apostasize and reject the Faith and become a non-catholic (in practice)? Yes, in the sense that they stop practicing the Faith.
1. Can such a one simply go to confession and rejoin the Church? No, they cannot. It's not that simple.
c. Can we consider a person who is formally excommunicated for schism (i.e. a catholic leaves and joins the Orthodox) a catholic? Yes and no.
1. But they are baptized? Yes, but still not a catholic.
2. Could they become a catholic? Yes, but there are still steps to take.
3. Are such steps EASIER and QUICKER than a pagan? Yes.
So, yes, in theory (based on baptism alone)...a person is "once a catholic, always a catholic".
But in practice (based on their lifestyle/religion choices)...a person can reject the Faith.
1. Catholic
2. Former catholic, heretic
3. Former catholic, schismatic
4. Baptized never-catholic/protestant
5. Baptized never-catholic/misc
6. Non-baptized pagan
"Once a catholic, always a catholic" presumes that #2 and #3 are "still catholic", in that they could MORE easily and MORE quickly convert than #4-6. But some would argue, in practice, that #2 and #3 aren't catholic anymore, being they've rejected the Faith.
Both are right. It depends how you look at it. Theory vs Practice.