43. What does it mean to believe in only one God?

To believe in the one and only God involves coming to know his greatness and majesty. It involves living in thanksgiving and trusting always in him, even in adversity. It involves knowing the unity and true dignity of all human beings, created in his image. It involves making good use of the things which he has created.

44. What is the central mystery of Christian faith and life?

The central mystery of Christian faith and life is the mystery of the Most Blessed Trinity. Christians are baptized in the name of the Father and of the Son and of the Holy Spirit.

45. Can the mystery of the Most Holy Trinity be known by the light of human reason alone?

God has left some traces of his trinitarian being in creation and in the Old Testament but his inmost being as the Holy Trinity is a mystery which is inaccessible to reason alone or even to Israel's faith before the Incarnation of the Son of God and the sending of the Holy Spirit. This mystery was revealed by Jesus Christ and it is the source of all the other mysteries.