In the past, there was a so-called "month of remorse" in some federal states, meaning church tax liability ended one month after the month of resignation. This applied to Berlin, Brandenburg, Bremen, Hamburg, Hesse, Mecklenburg-Western Pomerania, Saxony, Schleswig-Holstein, Thuringia.
However, the month of remorse has now been abolished to standardise church tax regulations across the country, meaning the resignation takes effect in the calendar month in which it is declared.
In Berlin, Brandenburg and Bremen, resignation is free of charge. In other federal states, you must pay between 10 and 60 Euro for the certificate of resignation.