help I have to make a whole new religion

Quote from CaptainJinxed >>

Quote from Agilemind >>

 My campaign is very, very storyline based, and this original religion has some of the answers to the big questions Solstara presents. For example, there are no dragonborns within the city, despite it being the capital of a country and have a wide variety of species/races within its walls. Additionally, they're actively digging into the origins of the religion and there may even be a prophecy involved. I'm very much in over my head, but I love this campaign to bits, and I want it to be as realistic as possible for my players. Not to mention, these gods are real whereas the current religion's goddess isn't. I hope this explains!

Well if you intend the religion to explain certain aspects of the world you have created you can't just take someone else's pantheon and plop it in. But if you want it to be realistic then there shouldn't be clear answers about something long gone. Even in our world most things from 500+ years ago are fundamentally unknown to some degree. Even big real-world events like the Black Death in Europe it is not completely clear why it happened. So the realistic case for a long dead religion is there to be contradictions, missing information, some stuff that is outright lies and propaganda, others that are overly zealous interpretations. The Vikings walked this earth only 1000 years ago, yet most of what we know about their religion is based on a handful of texts.

Even big real-world events like the Black Death in Europe it is not completely clear why it happened. 

Actually that is not necessarily true.  They (infectious disease historians/et.al.) have a good idea what happened and why it appeared beyond what any prediction would be.  They have a good idea of the mutations that happened along the path etc.  The unwashed masses are the ones that perpeturate the myth of not understanding why.