easy, simply go as far back as possible, and you will find that there has to be some kind of creator. even if you believe that a whole bunch of universes started other universes, it all has to come from one universe. where did that universe come from? the big bang? what made the materials for the big bang? and what made the materials for the materials for the big bang? it all has to come from somewhere.
tl;dr God's existence can be proven by simply going backwards and backwards, there has to be some kind of creator no matter the explanation