The universe exists, therefore He IS.
Stephen Hawking said that gravity could've created the universe, but his reasoning is faulty, because gravity itself requires the existence of the universe before it can exist (space, time, energy).
Some atheists ask, "Who created God?" which is equally faulty reasoning, because God is pure Source. This kind of flaw is easy to understand, because most humans are so used to thinking in terms of dichotomies, they have no concept for what the Buddhists called "paramita" or "perfection" -- the one-sided "coin" or "one hand clapping."
God is pure love without any of its opposite, ego (self-concern), and thus has no limitations.
When Peter stepped out of his boat onto the unsettled Sea of Galilee, he had that perfection of confidence called "Faith" which does not have its apparent opposite (doubt) embedded within it, like so many human attitudes, such as "belief."