"How is faith reliable when faith could be used to hold any position?" It can't, dear one, be used to hold any position. Faith must be based in something that is true. Faith has to have a way to be vindicated at some point. Atheism has no way to be vindicated, since one cannot know there is no God, much less prove it. One cannot have faith in atheism. That is blind belief.
"I could have faith that Zeus is the one true god and not the bible god." No, you couldn't. Nobody has dealings with Zeus, only with God, call him what they will.