Trinity Sunday, the Sunday after Whitsunday, is the only feast in the 1928 Book of Common Prayer calendar dedicated not to a redemptive event but to a mystery of the divine being: the eternal truth that the one God subsists as Father, Son, and Holy Ghost, three Persons in one undivided substance. It falls at the close of the great cycle of saving acts — Incarnation, Passion, Resurrection, Ascension, and the gift of the Spirit — and invites the Church to contemplate the God whose inner life those acts have disclosed.

The appointed Epistle from the Revelation of Saint John sets before the worshipper the heavenly throne room, where the four living creatures cry unceasingly: Holy, holy, holy, Lord God Almighty, which was, and is, and is to come. The trisagion is itself a Trinitarian acclamation, and the vision frames what the intellect cannot contain: that God's holiness is not a solitary attribute but the radiance of his triune love. The appointed Gospel from Saint John recounts the nocturnal conversation between our Lord and Nicodemus, in which Christ speaks of new birth through water and the Spirit, of the love of the Father who sends the Son, and of eternal life as the purpose of that sending.

The Collect of Trinity Sunday is among the most ancient in the Anglican inheritance, praying that the faithful, in acknowledging the glory of the eternal Trinity and worshipping the Unity, may by God's majesty ever be defended from all adversities. It unites doctrine and devotion in the pattern the Prayer Book always commends: right belief issuing in joyful and thankful prayer.