The idea would be that a perfect existence must exist as a part of being perfect, thus God (or some primary existence/force). That may sound handwavey, but let

The idea would be that a perfect existence must exist as a part of being perfect, thus God (or some primary existence/force). That may sound handwavey, but lets look at it the other way around - something existed "first" casually, because the universe exists. (Note I'm saying casually, I'm not ruling out something existing "forever" or getting into the existence or nonexistence of time, but something exists/existed without a cause that caused everything else.) Either that primary cause is the universe itself, for some reason, or it was something that caused the universe (force, God, whatever). It's unclear to me why the universe itself needs to exist, and to me the alternative that makes more sense that a perfect being needs to exist and caused the rest of it.