From early on we were told that the Trinity is a mystery, the loftiest and most impenetrable of mysteries. We were not expected to understand it, but simply to believe it. The Trinity is an absolute mystery in the sense that we do not understand it even after it has been revealed. It is a mystery in that it is “hidden in God and cannot be known unless revealed by God.”
To be sure God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout Old Testament. But his most 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.
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 never revealed by God.
The Doctrine of the Trinity is a summary statement of faith in the God of Jesus Christ. The focus of the doctrine of the Doctrine of the Trinity is the God who acts and is present in the “economy” (oikonomia meaning plan or administration) of salvation, especially in Christ. Christ is the revelation of God, the culmination of the economy of salvation in which the reality and mystery of God are made manifest. But it is said that God is a mystery.
The mystery of God is revealed in Christ to be the mystery of love offered for the sake of life, even amid rejection and death. Those who believe in Christ are offered the possibility of sharing in divine life and love.
 Richard P. McBrien, Catholicism (HarperCollins Publishers, New York: 1998), p. 316.
 Dei Filius, 4: DS, 3015.