Central to the Christian faith, distinguishing it from other monotheistic traditions, is the concept of God as one divine substance which comprises three distinct and co-equal persons: Father, Son and Holy Spirit. To express this mystery, Tertullian (c. 160-225) used the word trinitas, the Latin form of the Greek trias: ‘triad’.
How to explain this mystery of the Triune God with doctrinal precision, a task which gave rise to many heresies, is illustrated by the legend related of St. Augustine of Hippo, author of the philosophical treatise On the Trinity in fifteen books. On the sea shore he saw a child with a shell trying to empty the ocean into a hole in the sand. When he remarked on the impossibility of the task, the child replied that it was no more difficult than Augustine’s attempt to define the Trinity.
The classic definition was given in the creed promulgated by the First General Council of Nicaea in 325: “We believe in the Holy Spirit, the Lord and Giver of Life, who proceeds from the Father, who together with the Father and Son is worshipped and glorified.’
Prayers in honor of the Holy Trinity were composed in certain monasteries from the late seventh century onwards. There were also votive Masses, said at the discretion, or ‘choice” (Latin, votum), of the priest, notable examples being that composed in 800 by Alcuin, the renowned scholar, born and educated in York, who became adviser on liturgical matters to the Emperor Charlemagne. Most influential was the Mass of the Holy Trinity, celebrated in the eleventh century in the Benedictine monastery at Cluny, Burgundy and spreading from there to daughter-houses in other parts of France and in Spain.
Although tolerating these local celebrations, successive popes, following the lead given by Pope Alexander II (1061 – 73), resisted requests for a fixed day in the calendar for a Trinitarian feast on the grounds that “on all Sundays and, indeed every day, the Trinity is celebrated”.
The need to combat Unitarian heresies, notably those propounded by Albigensians and Waldensians caused Pope John XXII, in 1334, while in exile in Avignon, to consent to a universal feast on the first Sunday after Pentecost, following the commemoration of the descent of the Holy Spirit.
Trinity Sunday is known as an “idea feast,” since it commemorates not an event in salvation history but a particular aspect of doctrine. It remains one of the nine solemnities of the Church calendar.