St. Thomas Aquinas observed with St. Paul that there are things about the world and religion that we can know with certainty by reason and the 5 senses.  However, there are many things about God and the way He has given us to get to heaven that are beyond reason and our understanding.  This makes it necessary for God to come to us and revelal Himself so we can be saved from our present condition.  This is called Divine Revelation.