Atheists, just a few questions...?

There has been many different religions before Christianity and after Christianity that all have nearly the same dates and same stories. How am I suppose to believe that Christianity is the true religion if many religions before it had the same basic concept and story. Also in the bible god killed 33 million people the devil killed 5 ( god made him and 2 of them deserved it ) so if the bible was written by man from gods side of the story then until we get the devils side how are we suppose to know if the devil isnt the good guy and god is the bad guy.