Just to clarify this; if you are already paying church tax you need to write to the town's church authority to opt out. They'll write back confirming your request with a heart rendering "we'll miss you" letter.

If you are a business owner (AG or GmbH), you cannot opt out, you pay a pro rata rate which is a non-denominational payment. I'm not a fan, but the amount is not significant and if you have any sense, you should take your money out so that you can be taxed at the lower individual rate (where you can opt out) as opposed to higher corporate tax rates.