It is "the central mystery of the Christian faith and of Christian life." So says the Catholic catechism about the mystery of the Holy Trinity (no. 261). In Catholic use the word "mystery" always refers to a divinely revealed truth. This distinguishes it from a truth based solely on human science or reason.

One in Three

New Testament writers made many references to the Father, Son and Holy Spirit, but a precise definition of the Trinity did not arise until the second and third centuries with the teachings of Tertullian (c.160 - c. 225). He taught what Catholics believe today. There are three persons in one God. God is one because the three persons share in the same substance. In Catholic theology "substance" always means "essence" or "nature." That is why, for example, the Catholic creed calls the Son "consubstantial with the Father."

While alike in substance, however, each divine person remains distinct and unique. Tertullian was first to apply the term "person" to Father, Son and Spirit. His usage came from a Latin word meaning "mask." A Roman actor wore different masks to play multiple characters in a staged drama. By analogy, one God shows the created world three divine "faces": loving Father, redeeming Son and sanctifying Spirit.

The God Who Is

Yet to describe who God is by what God does in the created world tells only half the story. There remains the matter of identifying who God is in God's own self. "I am who am," the divine voice said to Moses on Mount Sinai. To resolve this problem, 20th-century Catholic theologian Karl Rahner coined two terms. He used "economic Trinity" for the God who acts in the world and "immanent Trinity" for the God who simply is.

Perfect Community

Leonardo Boff, contemporary Catholic theologian, suggests the Trinity is best understood as "the perfect community." His thought reflects St. Augustine's fifth-century teaching about the Trinity as a divine communion of Lover, Beloved and Love. That is, the Father begets the Son in infinite, loving self-outpouring. The Son returns that love in infinite self-outpouring. Their shared love becomes itself a third divine person, the Holy Spirit.

A geometric figure showing three interlocked, equal-sized rings can symbolize this perfect community. Visually, a kind of mutual sharing and interplay exists among the three rings. Each ring remains distinct while also interconnected with the other two. The theological term for this mutual sharing of divine life and love is "perichoresis."

Open Community

The Catholic church teaches that God invites every human being into the inner life of the Trinity. Receiving Christian baptism is the way a person accepts the invitation. However, baptism is only a beginning. Complete union with the Trinity grows over time and is perfected in eternity. It results from a lifetime dedicated to loving service of God and neighbor.