If you deny anything's existence, you must first presuppose its existence in order to argue against it. Therefore in a sense, it already does exist, or there would be nothing to argue against.
In this sense then, it's unquestionable that God and aliens exist.
Yep. We've proven two of the mysteries of life and it isn’t even dinnertime! But seriously, wouldn't it be great if there was a single argument or reasoning that by presenting itself as existing, would be enough to prove that it did? But life ain't that simple.
If, for now though, we accept the premise that God exists, we would probably want to accept that God conceived and created us? Classical philosophy believes God to be something of which nothing greater can be conceived. That means God is greater than that which is conceivable. This automatically puts God in the league of greater than the notion of God itself. But we already have the conception of God. Now, as we theoretically can't conceive this, the existence of God has to be an article of faith, as there is no proof. If there were proof then there would be no need for faith as you don't need to have faith in something that you Know. If it were known it would by definition be conceivable, and therefore it could not be greater than that conceivable, and this by definition could therefore not be God. When you can no longer conceive of it, that's when there is a slight chance that it might be true.
It's like a Zen paradox. In order to find something, you have to let it go; otherwise it is still with you. This is why anyone who claims to know God, simply can't. But the fact that we have a name for the concept, and this is understood by millions of people means that at least as a concept, God does indeed exist. And so the argument goes around and around like this until we turn ourselves inside out with it and devour ourselves... like an ouroborus.