Trinity Sunday, which this year falls on 27 May, was a surprisingly late addition to the liturgical calendar. We celebrate it on the Sunday after Pentecost (Whitsunday) and so, like Pentecost, the date on which it is celebrated changes from year to year because it stands in a fixed relationship to Easter, which varies annually. The high points of the first part of the year celebrate events in the life of Christ, the Crucifixion, Resurrection and Ascension, and the coming of the Holy Spirit to the Apostles (the Feast of Pentecost). But after Trinity Sunday we enter a period known as Ordinary Time, when the Sundays are counted by number: such-and-such a Sunday in Ordinary Time, or such-and-such a Sunday after Trinity. If Easter is as early as it can possibly be (March 22), then there are twenty-five Sundays ‘after Trinity’ before we get to Advent, which begins the new church year, but the later Easter is (and the latest possible date is 25 April), the number of Sundays ‘after Trinity’ is correspondingly reduced. The counter-balance is provided by the Sundays after Epiphany (6 January): there are more of these if Easter is late, and fewer if Easter is early. Before Trinity Sunday was established, the Sundays in the second half of the year were numbered from Pentecost or, as a common alternative, from the Octave of Pentecost – that is, the Sunday after Pentecost, which became known as Trinity Sunday when this was established in the fourteenth century.
Of course, the doctrine of the Trinity was central to Christianity long before this. Tertullian (died c. 240) wrote an extensive treatise on the ‘three persons’ of God, and although the subsequent development of Trinitarian theology diverged in some important ways from Tertullian’s ideas, his work was crucial in establishing the concept of the three-in-one and one-in-three of Christian doctrine, later confirmed in the great fourth century Councils of the Church, where it became a test of orthodoxy. Belief in the Triune God is, of course, declared in the Creeds, and it was always central to teaching and preaching. So important was it that, from the tenth century onwards, services were held in honour of the Trinity, although such services were a matter of local practice, not associated with an agreed and special day. In the eleventh century, Pope Alexander II was asked if a designated feast-day could be established, but he refused, on the grounds that the Trinity was honoured daily every time we use the phrase ‘In the name of the Father, and of the Son, and of the Holy Spirit’ — hence the title of this month’s article.
For the institution of a special day, universally recognised in the Western Church, we have to wait until 1334 and the papacy of John XXII. He was much involved in political and ecclesiastical intrigues, in the course of which his enemies had accused him of heresy — a very damaging accusation for someone in his position! His decision to introduce an agreed day when the Western Church would honour the Trinity was his counter-attack: a feast-day celebrating a tenet of belief that had long been the supreme test of orthodoxy. No doubt, in the midst of his intrigues, he was also aware that this liturgical innovation had what was for him the beneficial side-effect of winning over the still relatively new but already highly influential Order of Franciscans, who were very Trinitarian and keen to reflect this in liturgical practice.