Once again, The Economist.
Abuse and counterabuse
Around two-thirds of west Germans identify themselves as Catholics or Protestants. Christians who pay income tax hand over an extra “church tax” that accounts for two-thirds of church revenue. Germans are not devout: 4% of Protestants and 14% of Catholics in the west are weekly churchgoers. (My note: that’s 4% of Protestants, not the general population.)
…
Death and defection are slowly eroding the membership of both main churches. The drop in revenue is happening more speedily because those who leave tend to have relatively high incomes. Within 20 years the income of the Protestant church may shrink to two-thirds of its present level…