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