Introduction to Trinity Sunday:
Trinity Sunday, a moveable feast also known as Holy Trinity Sunday, is celebrated a week after Pentecost Sunday in honor of the most fundamental of Christian beliefs—belief in the Holy Trinity. We can never fully understand the mystery of the Trinity, but we can sum it up in the following formula: God is three Persons in one Nature. The three Persons of God—Father, Son, and Holy Spirit—are all equally God, and They cannot be divided.
- Date: The Sunday after Pentecost Sunday. See When Is Trinity Sunday? for the date in this and future years.
- Type of Feast: Solemnity.
- Readings: Deuteronomy 4:32-34, 39-40; Psalm 33:4-5, 6, 9, 18-19, 20, 22; Romans 8:14-17; Matthew 28:16-20 (full text here)
- Prayers: The Sign of the Cross; The Glory Be; The Athanasian Creed
- Other Names for the Feast: The Solemnity of the Most Holy Trinity, Holy Trinity Sunday
As Fr. John Hardon points out in his Modern Catholic Dictionary, the origins of the celebration of Trinity Sunday go all they way back to the Arian heresy of the fourth century. Arius believed that Christ was a created being, and in denying the divinity of Christ, he denied that there are three Persons in God. Arius' chief opponent, Athanasius, upheld the orthodox doctrine that there are three Persons in one God, and the orthodox view prevailed at the Council of Nicaea, from which we get the Nicene Creed, recited in most Christian churches every Sunday. (The Council of Nicaea also gives us a wonderful example of how a real bishop deals with a heretic; see my biography of Saint Nicholas of Myra for the whole story.)
To stress the doctrine of the Trinity, other Fathers of the Church, such as St. Ephrem the Syrian, composed prayers and hymns that were recited in the Church's liturgies and on Sundays as part of the Divine Office, the official prayer of the Church. Eventually, a special version of this office began to be celebrated on the Sunday after Pentecost, and the Church in England, at the request of St. Thomas à Becket (1118-1170), was granted permission to celebrate Trinity Sunday. The celebration of Trinity Sunday was made universal by Pope John XXII (1316-34).
For many centuries, the Athanasian Creed, traditionally ascribed to Saint Athanasius, was recited at Mass on Trinity Sunday. While seldom read today, this beautiful and theologically rich exposition of the doctrine of the Holy Trinity can be read privately or recited with your family to revive this ancient tradition.