St. Thomas Aquinas’ five proofs of the existence of God.
I assume you are referred to the Cosmological argument. Here’s how I understand the Cosmological Argument from Aquinas:
The world (universe) could not be the cause of itself. The world could not come from nothing. The world could not be an effect in an infinite series of causes and effects. Therefore, the world must be caused by a transcendent, cause (a cause independent of, or outside of, material universe). By definition, that transcendent cause is God.