Thomas Aquinas proved the existence of God through his "five ways" arguments. The uncaused causer is one of them. According to him, everything in the universe is caused by another thing. If we begin tracing the cause of things in the world, we would find out what puts them in motion. But as we continue tracing, we get to a point where the cause ceases. That point is the power that causes all things to motion but it has no cause that puts it in motion. It is the uncaused cause. That cause is God.