The Catechism in Questions & Answers

1. What is the source of our belief in God?

The source of our belief is in God Himself. He allows human beings to recognise Him: He “reveals” Himself.
The knowledge that God exists has been given by God Himself. God does not conceal Himself, but rather allows Himself to be recognised by human beings such that human beings can speak of God and believe in God.

“Because what may be known of God is manifest in them [human beings], for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse.”
Romans 1: 19-20