The whole noun phrase "symbol of your religion" is in Instrumental.
Inside it, "your religion" is in Genitive, because of "of".
Why "the symbol' rather than "a symbol'. I was marked wrong for 'a symbol'. Are religions supposed to have only one symbol each?
What is the symbol for your religion? was marked wrong. Another case of Duo trying to teach English. Come on, Duo, please accept this.