thats the thing about religion if you believe its real its real
Greek mythology itself is not a religion. It is part of the religion called Hellenic Polytheism, or Hellenic Paganism.
And yes, it IS a real religion for those of us who follow it. Just like I know that Christianity, Buddhism, and all the others are real religions, even though I don't follow them does not make them any less real. And just because others may discount the validity of any given religion or spiritual path does not make it any less real or valid.