The word Trinity comes from Trinitas, a Latin abstract noun that means “three-ness,” “the property of occurring three at once,” or “three are one.” The Greek term used for the Christian Trinity – “Trias” means “a set of three,” and has expressed by the English word Trinity. This word literally means “a unity of three.”
It is the truth that in the unity of the Godhead there are Three Divine Persons, the Father, the Son, and the Holy Spirit. These Three Persons being truly distinct one from another are co-eternal persons and are of one indivisible Divine essence.
It is only in the light of the divine life, “One God in Three” that all the other realities of revelation can fully be appreciated. It is in the light of the Blessed Trinity that the essence of Christian life is clarified. It is only when we begin to understand the implications of a personal Trinity in God that we can grasp the importance on union with Christ, activated by faith and baptism, and consummated in the possession of the beatific vision. The mystery of the Blessed Trinity is the most fundamental among the revealed mysteries of Christianity (ccc 234).
There are two approaches to our attempts to understand more about and reflect over the doctrine of Trinity.
One way is to speak about Immanent Trinity (ad intra); it is the esoteric exposition of God’s inner life. It is rational speculation aided by reason and faith about the self-relatedness of the Father, the Son, and the Holy Spirit. This is also called the Ontological Trinity. It speaks of the essence (Jn 1, 1-2), nature or attributes of the Trinity. Or more simply who is God.
The second way is to speak of the Economic Trinity (ad extra), as we see how salvation is worked out by three divine persons. The approach is more consistent with the Bible, Creeds, and Liturgy. This method of reflection makes it possible to think of the Triune God in relation to our redemption. In short ‘what does the Trinity have to do with our salvation?’ When we describe the acts of the Triune God with respect to the creation, history, salvation, our daily lives, etc, we describe the Economic Trinity. This approach refers to how Trinity operates within redemptive history as we think of the roles or functions performed by each of the persons of the Trinity (Karl Rahner, in his essay The Trinity).
These two approaches are complementary not contradictory. The economic Trinity reflects and reveals the ontological Trinity. The integration of these two ways of knowing God shows how the Trinitarian pattern of salvation history is correlated with the eternal being of God.