Cause and effect are dependent upon time and our relationship with time.
From a non-temporal (timey-wimey) point of view there is no cause and effect.

A question like 'what existed before time' is meaningless because we'd need a preexisting temporal frame of reference in which to make the observations.
It's only our relationship with time that makes us think in temporal terms - reality isn't dependent on our limited point of view.
The big bang isn't really an historical event since it didn't 'happen' in time. It's just one end of a temporal object viewed from a very limited human perspective.

The existence of God or otherwise here is kind of irrelevant, because even if we had proof of the existence of God it still wouldn't be clear what we should do about it  - should we sing hymns? light incense? sacrifice goats? wear funny hats?

I can prove the existence of the Sun, but it's mere existence isn't enough to make me want to worship it.