I. "IN THE NAME OF THE FATHER AND OF THE SON AND OF THE HOLY SPIRIT"
The Trinity is a mystery of faith in the strict sense, one of the "mysteries that are hidden in God, which can never be known unless they are revealed by God". To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel's faith before the Incarnation of God's Son and the sending of the Holy Spirit.
Source: Catechism of the Catholic Church (Vatican.va)