OK, so if a heretic, schematic aren't members of the church (does this include those excommunicated?) Then why aren't they re-baptised when they recant their errors? Honest question.
I think we must distinguish what membership means. Baptism is formal entry into the church; you are spiritually reborn. Now, we know that cant be undone, which is why its called an indelible mark.
But, in the case of Martin Luther...hes a confirmed heretic. Hes outside the church. But isn't he "outside" in a different way than an unbaptised pagan? There has to be a difference. If not, then all "outside" would have be re-baptised, which isn't the case.