Motion
In the universe, there are objects in motion, each object in motion must have been caused to move by something else. Because nothing is infinite, there must have been a first motion or "prime mover" which was unmoved, this prime mover began all movement without bieng moved, Aquinas stated that the "prime mover" must have been God.

Causes
Nothing can be the cause of it's self, as it would have to 'exist' before bieng caused. This of course is impossible, so like with motion there must have been a first "un caused" cause, that started a chain of causes. This is God.

Contingency
There must have been a time when nothing existed, the cause of the universe would have to be outside of the universe and must have always existed. There muse have been something to cause the universe to exist, this is God.

Of course, this can also be applied to the big bang. So you could say

God = the Big Bang.