Mark,
it all comes down to what the Church is. Christians know it is the Body of Christ, outside of which there is no salvation. Christians know it was established by Jesus Christ and no man is authorized to start another one. Christians know the Church has been here since 33ad and will be here until the second coming of Jesus at the end of the world. Christians know there is One Lord, One Faith, One Baptism, One Church, One Eucharist, One canon of Scripture.
for those who think the Church is like the moose lodge or rotary club, they don’t believe any of the above.
I think that it is more basic even than this. I think that it is a third grade fantasy about being popular with the teacher.
Those who have been validly baptized belong to the Catholic Church by a sacramental bond even if they are not card-carrying members.