Perhaps the Germans could create a provision in their tax law that requires everyone to pay their religion tax, but allows each individual to select each year which religion they wish to send their funds to that year, and allows atheists to select a non-religious charitable fund, an organization for atheists and agnostics that holds meetings, or some sort of national network of community centers (i.e. free places for people to meet and hang out that might serve food, have a small library or gym, computer terminals, chess boards, etc..) instead of a religion? That way you avoid this stuff about trying to prove whether or not someone is religious and/or what religion he is. Everyone pays the tax, everyone decides to where to direct it individually each year, and there are non-religious options. That's the way I'd do it if they feel they need to get the government involved in funding religion at all.
Doing things the way I propose would not only end these government inquiries into people's faiths and people attempting to dodge the tax by claiming no religion when they in fact have one, but it would also allow people to redirect their funds in protest if their church did something they didn't like. For example, if the Lutheran church decided to support the death penalty very publicly and a member was opposed, he or she could instead send their tax money to a charity, or to a different church, and send their church a letter telling them what they did and why. Also, someone who was not a member of any church, but admired a public stand taken by a church, could designate that their taxes go to that church during that particular year without joining the church and send them a letter letting them know why.
I should clarify that I realize that isn't the system there now. It's just a proposal I thought up that might make the system a bit more palatable for people, and save government resources that are going to trying to figure out which religion everyone is.
