On Trinity Sunday we proclaim the mystery of our faith in the triune God: Father, Son and Holy Spirit, One-in-Three and Three-in-One.
The celebration of Trinity Sunday began among Western Christians in the 10th century and developed slowly until it was formally established on the Sunday after Pentecost by Pope John XXII (1316-1334).
An excerpt from the Companion to the Book of Common Worship (Geneva Press, 2003, 149-150)
Unlike other festivals in the church’s liturgical calendar, Trinity Sunday centers on a doctrine of the church, rather than an event. It celebrates the unfathomable mystery of God’s being as Holy Trinity. It is a day of adoration and praise of the one, eternal, incomprehensible God.
Trinity Sunday, in a sense, synthesizes all we have celebrated over the past months which have centered on God’s mighty acts: Christmas-Epiphany celebrating God’s taking flesh and dwelling among us in Jesus Christ; Easter celebrating Christ’s death and resurrection for us; Pentecost celebrating God the Holy Spirit becoming our Sanctifier, Guide, and Teacher. It is, therefore, a fitting transition to that part of the year when Sunday by Sunday the work of God among us is unfolded in a more general way.
The triune God is the basis of all we are and do as Christians. In the name of this triune God we are baptized. As the baptized ones we bear the name of the triune God in our being. We are of the family of the triune God. We affirm this parentage when, in reciting the creeds, we say what we believe. Our discipleship is rooted in the mighty acts of this triune God who is active in redeeming the world. The triune God is the basis of all our prayers — we pray to God the Father, through Jesus Christ, by the Holy Spirit. The Trinity holds central place in our faith. …
In celebrating Trinity Sunday, remember that every Lord’s Day is consecrated to the triune God. On the first day of the week, God began creation. On the first day of the week, God raised Jesus from the grave. On the first day of the week, the Holy Spirit descended on the newly born church. Every Sunday is special. Every Sunday is a day of the Holy Trinity.