Catholicism is monotheistic: it acknowledges that God is one, eternal, all-powerful, all-knowing and omnipresent. God existed prior to His work of creation (that is, everything which is not God, and which depends directly on him for existence) and yet is still present intimately in His creation. Faith in God’s existence is the most fundamental Catholic belief, and in the First Vatican Council the Church has taught that, while by the natural light of human reason God can be known in his works as origin and end of all created things, God has also chosen to reveal himself and his will supernaturally in the ways indicated in the Letter to the Hebrews 1:1–2.

Catholicism is also Trinitarian: it believes that, while God is one in nature, essence, and being, this one God exists in three divine persons, each identical with the one essence, whose only distinctions are in their relations to one another: the Father’s relationship to the Son, the Son’s relationship to the Father, and the relations of both to the Holy Spirit, constitute the one God as a Trinity.

A Catholic is baptized in the name (singular) of the Father and of the Son and of the Holy Spirit — not three gods, but one God subsisting in three Persons. While sharing in the one divine essence, the Father, Son, and Holy Spirit are distinct, not simply three “masks” or manifestations of one Person. The faith of the Church and of the individual Christian is based on a relationship with these three Persons of the one God.

The Catholic Church believes that God has revealed himself to humanity as Father to his only-begotten Son, who is in an eternal relationship with the Father: “No one knows the Son except the Father, just as no one knows the Father except the Son and those to whom the Son chooses to reveal Him.”

Catholics believe that God the Son, the Divine Logos, the second of the three Persons of God, became incarnate as Jesus Christ, a human being, born of the Virgin Mary. He remained truly divine and was at the same time truly human. In what he said, and by how he lived, he taught us how to live, and revealed God as Love, the giver of unmerited favors or Graces.

After Jesus’ crucifixion and resurrection, his followers, foremost among them the Apostles, spread more and more extensively their faith in Jesus Christ with a vigor that they attributed to the presence of the Holy Spirit, the third of the three Persons of God, sent upon them by Jesus Christ.