One thing I've wondered, if your parents register you under a certain church, do you always stay registered under that upon becoming an adult unless you specifically go through the process to de-register, or is it something you have to continuously re-register as?
Religionsmündigkeit, or the legal ability to choose yourself what religion to adhere to, is obtained by turning 14 years old. Not
entirely certain, but I'm relatively sure you remain a member of one of the churches unless you explicitly de-register upon becoming an adult (14 years old for the purpose of this issue).
This isn't true. Church taxes are
owed if you are a member of a church that taxes you through the state. The only way to avoid paying them, as far as I know, is to renouce your membership in that church. (Which involves going down to city hall to officially de-register, and what's really freedom hating, pay a fee in most states.)