According to a family tree I found on MyHeritage, a cousin of mine, born Jewish in Vienna, was married in 1917 to a Christian in St Servatius Cathedral in Siegburg. I have little experience with German records and no experience with records for Catholic marriages.  How would one obtain a record?  My cousin had a very common name so there is a reasonable likelihood of an error in the tree. 

Michael Gordy
Takoma Park, Maryland, USA

German has had obligatory civil marriage since 1876, so you should ask the city archive of Siegburg about the record:
https://www.archive.nrw.de/en/stadtarchiv-siegburg
--
Andreas Schwab, Montreal, Canada

Dear Michael,
in addition to what Andreas Schwab wrote: In Germany, church records are also available, mostly in the local parishes and/or in the higher-level church record archive.
Wolfgang Fritzsche, prof. genealogist, Germany, near to Mainz/Wiesbaden