The Cosmological Argument is a theory put forward by St. Thomas Aquinas. In the Cosmological Argument, he said that he believed that all things in the world were caused to exist by something. The universe exists, so there must be something that caused the universe. He believed the first cause to be God. He said that God is the only thing that was not caused by something else and He created the cause of existence.
He also said that the first piece of proof of God's existence is the fact that the Universe exists. The Universe could not have come from nothing. Something cannot come out of nothing. Because of this, the Universe was caused to exist by something that existed before it. He said that this could only have been God.