So if membership in the church can only be had by baptism, then why isn't baptism required AGAIN, if one is formally declared a heretic or excommunicated? It only makes sense if there are different meanings to membership.
Example: We get baptised, we all learn we're now "children of God". OK, but if we commit mortal sin, the catechism also says we are now "cut off from God." Does being in mortal sin mean you aren't a child of God anymore? Of course not.
Point is, being "outside of the church" means different things. For a pagan, it means they aren't in union with the church, and never were. For a heretic/excommunicated person, it means they are "cut off" from the Church, temporarily (in theory), just like mortal sin. I just don't see how it means membership is revoked; else re-baptism would have to happen.
Pax, the Sacrament of Baptism is
the gateway to the Church and leaves and
indelible mark. It is
necessary but not
sufficient for membership.
For instance, someone born into a Protestant sect, if validly baptised during infancy, will be a member and subject of the Roman Pontiff, until, coming of Age, he will embrace some heresy (becoming a heretic), for instance his parents', or maybe turn away from the faith and disbelieving (apostate). Possibly, he will also reject the Pontiff, therefore becoming a schismatic.
One of the Marks of the Church is that is One and consequently she has Unity of Faith.
Formal Heresy rescinds this bond, by falling away even from only one article of Faith, it is as if you lose the whole.
The fact Heresy, Schism and Apostasy sever one from the Church is declared many times in magisterial docuмents, such as:
“For not every sin, however grave it may be, is such as of its own nature to
sever a man from the Body of the Church, as does
schism or
heresy or
apostasy.” in Mystici Corporis Christi.
Likewise that heretics are NOT part of the Church is (relatively) frequent when dealing with the topic.
Both heretics, apostates and schismatics are non members, just like a Hindu for instance, only they have some sort of relation with Church by means of Baptism.
I heard it referred as "members in potentia" (all baptised people) vs "members in actu" (members of the Church=Catholics proper).
But we know the Church is a Body, and United in Faith, therefore they are outside.