Question asked: 'What if no religions are correct, but there still is a God?'
Nothing much would happen, I venture. It would probably mean that said God doesn't know or care about us, and that probably would mean that there's no afterlife to speak of.
Or perhaps this God is so nonrestrictive or forgiving that his/her/its religious guidelines are so vague and generalized they are to an extent present in all religions. In that case, most of us might get a free ticket just by existing. That creates a fuzzy warm feeling in my tummy.
Other explanations seem problematic(sadistic deity, powerless deity), but the ramifications of this question won't really be an issue until we die, anyway.