He will glorify me, because he will take from what is mine and declare it to you. Everything that the Father has is mine;for this reason I told you that he will take from what is mine and declare it to you."
-- Jn 16:12-15
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.
-- CCC 237
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."
-- CCC 253