You were mistreated and abused your whole life. Everyone was disgusting they made you feel worthless and prayers aren't even being answered that is quite your reason why you ended up as an atheist. One day you came home from school ignoring your drunk arguing parents in the kitchen when you noticed a silver star on your desk that wasn't there before. You touched it

"Hello mere mortal! I was sent by the depths of hell and by the god of the thrones of heaven. I am your guardian demon! Acheron!"