Through God's actions, He reveals His identity and Trinitarian being to us, allowing us to better understand Him as we engage in prayer, study of the catechism and scripture, and participation in the sacraments. However, His inmost being as Holy Trinity remains a mystery that cannot be accessed through reason or faith alone.
The central mystery of the Christian faith and life is the Most Holy Trinity. It is a mystery that is hidden in God and can only be known through divine revelation.
The Trinity is the central mystery of the Christian faith because it's the mystery of God in himself. Each time the sign of the cross is made, the Trinity is highlighted as the three divine persons - the Father, the Son, and the Holy Spirit - in one divine being are represented.
God reveals Himself through his actions, the better we understand his actions, the better we understand his identity. However, while traces of his Trinitarian being are evident in his works and the Old Testament, his Holy Trinity is a mystery that remains beyond human comprehension.