Everyone is subject to the German “church” tax, it’s a percentage of each person’s income and they simply check which category they prefer. Catholic is the biggest, then Lutheran, but there are also categories for atheists and some sort of greenie-gaia-transcendentalists, or maybe those are lumped into one category!
But my original question hoped to ask that if the Lutherans are also subject to the tax, are their numbers also declining at the same rate as the Catholics? If the Lutherans are not leaving in droves, then the decline amongst the Catholics has nothing to do with the church.