If you don't have a God -- or some such underlying fundamental principle -- to authorise the tenets of your belief, then youcan't justify your beliefs as being essential, correct or necessary. I realised this a good many years ago, before my faith in God "wore off". To me, using a Creator to explain where everything came from doesn't answer the question: it's simply adding an extra step to the conception that the universe is everything there is and there isn't any "before" or "outside" or "other". (I look forward to being proved wrong on this, by the way!)
Without God to back up what I think, there is no meaning to my thoughts, and no meaning to existence. I suppose I could get used to that idea, but it will take a long time ... instead, I can say (I have to say) that it all has meaning because I choose to give it meaning. But this means I can't believe that my thoughts are more correct than yours, or vice versa. I'm content to accept that no-one whose beliefs I know of (by which I mean the major religious faiths) has the full and exact answer; but that many have a set of beliefs that are valid for part of the answer. Remember the story of the wise men and the elephant? They'd never seen one, but had the oportunity to examine one without being able to see it: the first said it was like a huge muscular snake, the next said it was like a great leathery wall, and the last said it was like a length of rope; all equally valid, but only within their limited scope.
My brain starts to hurt when I've got this far, so I'll leave off philosophising now; but I hope I've got my point -- however pointless! -- across.