who told you there is god ?
People.
who told you there is a devil ?
People.
Has anyone ever seen either of them other than the few very chosen ones in the history od mankind ?
Nope.
Has god remained the same since the begining of civilization ?
Nope, our definition of god keeps evolving as human civilizations keeps developing(from worshiping mother nature, to idol worshiping, to spirituality).
So what is god ?
Nobody knows, even the pope dun know. We just go by what we are told. 
Do prayers work ? 
Only god knows.... lol !!