My first point is always that none of us can ever know if there is a God or not, for the simple fact that it is impossible to know the nature of reality.
The "Matrix" series has been a recent popularization of the Hindu concept of "Maya", which basically says that all we know is a cleverly crafted illusion. I'm not saying this is true - simply that there's no way we could ever tell. All we know is by inference - the sun's always come up, gravity's always pulled us down, and water's always come out of the tap when you turn the faucet. That doesn't mean it won't change.
Given that, even if we were to come face to face with an immensely powerful being claiming to be God, we would still have to choose to believe if this was God, a hallucination, or simply another Wizard of Oz.
It's especially complicated for us. The ancient Hebrews, for example, had a simple, limited, tribal god. He was their sheperd - as opposed to the gods of others.
Nowadays the position is harder to fill. God, as we have defined the concept, is the creator of the universe, and both omnipresent and omniprescient. Quite the big shot, and not a job for just any giant glowing entity.
Everyone must choose to believe in something, and I've chosen to believe that the reality I perceive is largely the true one, and that the Scientific Method is gradually refining our knowledge of this reality.
From this, I get two facts. First, it is quite easy for complexity to arise from simple first causes. Second, intelligence is an extremely complicated thing.
So my choice of cosmologies is that either (1) A few simple, mindless first principles came into existence, and through interaction brought about the complexity we see today, or (2) An immense intelligence suddenly came into existence, and designed all we see.
I'm sorry, but while the first option is astounding and awe-inspiring, it also fits better and better with everything that's been observed.
And door #2 is just silly. I mean, "poof!", something more complicated than the universe comes into being, and is why the universe is so complicated.
Again, I don't know. Silly things happen all the time, and my fundamental beliefs could be wrong.
But I do know that if He exists, and designed the human race, He has a hell of a lot to answer for.