The Catholic Church does not believe that God must reveal himself in order for us to know that he exists. The reason for this is that God’s existence can be determined from philosophy. This is evidenced by the fact that many people throughout history have determined that God exists even though they did not have any form of revelation to draw from. Aristotle was one of these People. Using philosophy (and a little bit of science) Aristotle concluded that there must be one God, an “unmoved mover,” who is responsible for all subsequent causes in the universe.
But there are many things concerning this God that philosophy cannot teach us. For these things, the Church needs God to communicate His self to us. The Church needs revelation.
The most important doctrine that God has revealed to the Church is the doctrine of the Trinity. In its simplest terms, this doctrine states that the One God of the Universe is a communion of three persons: Father, Son, and Holy Spirit. In other words, God is a family. In this family, there is a Lover (the Father), one who is Loved (the Son), and the Love between them which is so real that it is actually a distinct and eternal person (The Holy Spirit).
The doctrine of the Trinity has always been the central doctrine of the Catholic Church. Whenever Catholics gather for prayer we begin by signing ourselves “In the name of the Father, and of the Son, and of the Holy Spirit.” The Catholic Church has always followed the command of Jesus to baptize in a Trinitarian form. Our most ancient members including Justin Martyr, Irenaeus, and Origen all believed in the Trinity. Whenever a person of influence has questioned the doctrine, such as an early fourth century priest named Arius did, the Popes and Bishops in union with him have always held their ground. In fact, it these Bishops were the first to use the word Trinity to describe the doctrine of three persons in one God.
The Catholic Church is all about the Trinity. The Catholic Church received this doctrine as a gift from God and the Catholic Church assists Christians everywhere in understanding this doctrine right up to this very day. We believe it, we profess it, and we coined the term.