This Sunday, 26 May, we celebrate the solemnity of the Most Holy Trinity. It is one of the few feasts that celebrates a doctrine instead of an event. The liturgical color is white.
Pope Gregory IX instituted Trinity Sunday in 828 AD. This day is dedicated to the Christian belief in the Trinity – Father, Son and Holy Spirit – and falls on the Sunday following Pentecost each year. The celebration of Trinity Sunday was made universal by Pope John XXIII.
The origins of the celebration of Trinity Sunday go all the 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 one 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 Nicea from which we get the Nicene Creed. For many centuries, the Athanasian Creed was recited at Mass on Trinity Sunday.