Follow-up question: to what extent was early Christianity just another exotic mystery religion from the East? And if Christianity was, indeed, 'just' a mystery religion, why did it go on to outshine the others?

Since this thread has mostly talked about Christians, can I ask this related question: Was Mithraism the biggest mystery cult, and how much do we know about it? And if it wasn't the biggest mystery cult, why is it the most famous?