Catechism of the Catholic Church (1992)
No. 234:
The mystery of the Most Holy Trinity is the central
mystery of Christian faith and life. It is the mystery of God in himself. It is
therefore the source of all the other mysteries of faith, the light that
enlightens them...
No. 237:
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 Israels faith before the Incarnation
of Gods Son and the sending of the Holy Spirit.
No. 253:
The Trinity is One. We do not confess three Gods, but one
God in three persons, the consubstantial Trinity. The divine persons do
not share the one divinity among themselves but each of them is God whole
and entire: The Father is that which the Son is, the Son that which the
Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature
one God. In the words of the Fourth Lateran Council (1215), Each of the
persons is that supreme reality, viz., the divine substance, essence or
nature.