The Argument from Change
The world of our experience is not static, not abstract unchanging theoretical stasis, but changes and grows. Things come into being and go out of being. Until something comes into being, it doesn't exist. It can't make itself exist. It can't cause itself to exist. You, me, lawns, term papers, dreams, mountains, written constitutions. Whatever. Whatever has no being is unable to bring about being. Whatever doesn't exist can't bring about that it exists.
About things that change, they already have existence, i.e., they already have being. Acorns turn into oaks, but they actually exist as acorns. (I'm talking about Act and Potency here, for the Aristotelians amongst us.)
The thing that changes cannot bring about that it becomes what it is not, for it is not existing as that thing--it is only potentially in that changed state, but not actually in that changed state. Nothing can give itself what it does not have. To become what you are not, you have to have input from a being that has that existence actually.
Everything changes, both the parts and the whole of everything (the universe?, are we using that word these days?). That means that everything inside the universe, and the universe as a whole, needs something actual to bring about the potential state to move into actuality.
This is what we call God, this is one of the aspects of God by which we refer to him in the Christian tradition.