- Everything must have a cause, so something must've been the very first cause; this is god,
- Goodness exists; if goodness exist, there must be some immutable standard by which actions are judged so that goodness exists in the first place. This is god,
- and so on.
Perhaps the best reason to believe in a god, and the one which no one can really argue against, is "Because I want to."
It's simple — it makes no unnecessary assumptions.
It's truthful — no one can deny your own wants.
It's personal — it relates god-belief to a wholly personal sphere which keeps it away from science, education, healthcare, and public policy.
Through the millennia as we learned more about the world and failed to find our gods reigning high on their mountains, theistic religions lost a lot of appeal, leading to their leaders inventing more complex reasons to continue to believe, to bolster the veracity of their religion against secular alternatives.
But ultimately? "Because I want to" should be enough. Anything more seems to entirely miss the point. What do you think?