"In che religione credi?"
Traduzione:What religion do you believe in?
C'è una spiegazione molto valilda che potete trovare nel link in calce e che riporto in sintesi:
Believe means to accept that something or someone’s words are true. You can believe a person, or something that a person said:
I believe her. She is always honest. Do you believe her story? It sounds fishy to me.
Believe in is a little different. Believe in means to have faith that something exists. You believe in something because you have faith that it exists, even though you may never have seen or experienced it:
Do you believe in ghosts? Many people in the world believe in God.