To add a little information to what the parent poster has sad. The state collects church tax for the Roman Catholic Church and Lutheran Protestant Church. The reasons for this arrangement go back to the early 19th century when the state appropriated the land that formerly belonged to the church. This was meant as a compensation for losses incurred by the state when Napoleon occupied the Western Rhineland. Don't ask me how that makes sense.
So, if you belong to either of those two denominations, the state will collect a church tax from you and pass it off to the church. To get out of this you have to go to the Amtsgericht (local court) and declare that you're not part of the respective church community anymore. You don't have to declare that you're an atheist, though.
You right, this arrangement is stupid but it's almost 200 years old and not likely to change anytime soon. Those who have to pay the tax don't seem to mind. Interestingly, it was never meant to be permanent. We have a saying here in Germany: "Provisorien halten am längsten." Literally translated it means that provisional arrangements last the longest.