You select the church it goes to, not everything is included, just the ones that collect their taxes through the state. It's not all christian either, think there were some jewish ones and probably more. Though pretty sure muslim is not there, they were trying to do it but never got organized or something along the lines.
The church is supposed to finance itself... so you don't pay a tax to a church if you're not in that church. It's something like 30% catholic, 30% protestant, 30% nothing (mainly from previously Eastern Germany), 10% other.
The state still spends regular tax money on some church-related things... some hospitals for sure, some schools, but not on a religious basis.







