The Centrality of the Trinity
The Catechism of the Catholic Church describes the Trinity as the "central" mystery of faith. Here let me quote the Catechism in full.
"The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith" [GCD 43.]. The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men "and reconciles and unites with himself those who turn away from sin" [GCD 47]" (CCC 234).Why is the Trinity so important? Well, theologically the Trinity is unique. Specifically, while all the other mysteries of faith describe what God does for us, the doctrine of the Trinity alone teaches us who God is in his deepest mystery. As the Catechism stresses: "It is the mystery of God in himself."
In fact, the centrality of the Trinity in Catholic theology and spirituality is evident from its prominence in the most recognizable of all Catholic prayers: the Sign of the Cross. While signing themselves with the cross, Catholics pray: "In the name of the Father and of the Son and of the Holy Spirit."
Given its significance then it is no surprise that the Trinity has its own feast day. But why do Catholics celebrate the doctrine of the Trinity this Sunday.
Theologia and Oikonomia
How is it that we know about the doctrine of the Trinity? Indeed, there is no single verse in Scripture which states it succinctly--i.e., "there are three divine persons who share on divine nature". How is it that God reveals the truth about his Triune life to us? Once again, let me turn to the Catechism.
"The Fathers of the Church distinguish between theology (theologia) and economy (oikonomia). 'Theology' refers to the mystery of God's inmost life within the Blessed Trinity and 'economy' to all the works by which God reveals himself and communicates his life. Through the oikonomia the theologia is revealed to us; but conversely, the theologia illuminates the whole oikonomia. God's works reveal who he is in himself; the mystery of his inmost being enlightens our understanding of all his works. So it is, analogously, among human persons. A person discloses himself in his actions, and the better we know a person, the better we understand his actions."In other words, God reveals who He is in His deepest mystery, i.e., His Trinitarian life, through what he does in salvation history (e.g., the oikonomia).
In particular, God is understood to reveal who He is in His deepest mystery in fullness in the coming of Christ and the Spirit.
First, God the Father sends the Son in the Incarnation:
"He who receives you receives me, and he who receives me receives him who sent me" (Matt 10:40).Next, the Son returns to the Father, i.e., the Ascension:
"he who receives any one whom I send receives me; and he who receives me receives him who sent me" (John 13:20).
"[Jesus in prayer to the Father]: I have given them the words which thou gavest me, and they have received them and know in truth that I came from thee; and they have believed that thou didst send me" (John 17:8).
". . . now I am going to him who sent me" (John 16:5).Finally, the Father and the Son send the Spirit, i.e., Pentecost.
"I came from the Father and have come into the world; again, I am leaving the world and going to the Father" (John 16:28).
"I tell you the truth: it is to your advantage that I go away, for if I do not go away, the Counselor will not come to you; but if I go, I will send him to you" (John 16:7).What happens in salvation history is understood as reflecting the inner Triune life of God--i.e., what God does (oikonomia) reflects who God is (theologia). Thus, the sending of the Son and His return to the Father reflects the Triune life of God: the Son proceeds from the Father and the Spirit proceeds from the Father and (through) the Son.
". . . the Counselor, the Holy Spirit, whom the Father will send in my name, he will teach you all things, and bring to your remembrance all that I have said to you" (John 14:26).
Hence, after celebrating Pentecost we focus on the Trinity--the sending of the Spirit in a sense completes the revelation of Triune life of God.
In short--and obviously much, much more could be said--that's why we celebrate Trinity Sunday after Pentecost.