Many German church records are available in online databases. Unfortunately they are not searchable, but only available as scans you need to go through manually. You’ll find the Catholic church’s database at https://data.matricula-online.eu/de/ (free to use; incomplete – more records are constantly added) and the Evangelic church’s database at https://www.archion.de/ (paid service; also includes many military church records as well as many records of Jewish communities). You can ask me when you need help finding something.
Generally useful links
When you come to my site, you probably already know a lot about genealogical research. So I’ll only put a few links together that you might not have considered yet as being useful:
Historical address books from Berlin: https://digital.zlb.de/viewer/cms/155/
Jewish holidays for any given year: http://jewishholidaysonline.com/ (very useful when you have an undated letter in which a holiday is mentioned)
A detailed timeline of World War I: https://www.historyplace.com/worldhistory/firstworldwar/index.html
A detailed timeline of the Holocaust, with all prohibitions etc.: http://www.historyplace.com/worldwar2/holocaust/timeline.html