Holy Trinity Sunday is celebrated on the first Sunday following Pentecost in most of the liturgical churches in Western Christianity. It is a solemn celebration of the belief in the revelation of one God, yet three divine persons. Yet, It was not uniquely celebrated in the early church!
But as with many things the advent of new, sometime heretical, thinking often gives the Church a moment in which to explain and celebrate its own traditions; things it already believes and holds dear. In the early 4th century when the Arian heresy was spreading, the early church, recognizing the inherent Christological and Trinitarian implications, prepared an Office of Prayer with canticles, responses, a preface, and hymns, to be recited on Sundays to proclaim the Holy Trinity. Pope John XXII (14th century) instituted the celebration for the entire Church as a feast; the celebration became a solemnity after the liturgical reforms of Vatican II.
In the shadow of Pentecost and the dramatic coming of the Holy Spirit, the following week seems a place to fitting place to pause, as it were, and place it all in a context of salvation history. Perhaps that is why the second reading was selected and says it so well: “The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with all of you.” (2 Cor 13:13). It ties together the first reading and psalm which point to the working of God before the coming of the Christ as well as our gospel reading, a short passage from the John 3:16-18:
16 For God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life. 17 For God did not send his Son into the world to condemn the world, but that the world might be saved through him. 18 Whoever believes in him will not be condemned, but whoever does not believe has already been condemned, because he has not believed in the name of the only Son of God.
While it seems a fitting set of readings, the gospel passage itself is but a small portion that sounds as though a capstone-like statement of the role of Jesus in our salvation. But the “capstone” is part of an early scene within the Johannine narrative in which the beginning of the revelation of the identity, mission and purpose of Jesus’ life – and ultimately death and Resurrection.