On Trinity Sunday many Christians in the United States remember and honor the belief of an eternal God, consisting of the Father, Son and Holy Spirit. Many churches have special prayers on Trinity Sunday. The Athanasian Creed, named after St Athanasius, Archbishop of Alexandria, is recited in some churches on Trinity Sunday because of its strong affirmation of the Triune nature of God.

Churches and choirs across the United States have planned special activities and events centered on Trinity Sunday. These include:

• Choral vespers concerts.
• Special church services focusing on the Holy Trinity and featuring music by the trumpet and the pipe organ.
• Special awards given at Trinity Sunday church services.
• Meals and social events after church services.
• Evensong services.

The ordinal Sundays following Trinity Sunday are focused on Christian growth and discipleship after a long period of emphasis on the life and ministry of Jesus. In the Eastern Orthodox and Eastern Catholic churches, the Sunday of Pentecost itself is called Trinity Sunday.

...Pope Gregory IX instituted Trinity Sunday in 828 CE. This day is notable for being the only major Christian festival that celebrates a doctrine of the church rather than an event in its sacred history. It is dedicated to the Christian belief in the Trinity – Father, Son and Holy Spirit – and falls on the Sunday following Pentecost each year. The belief of one God in three elements is a distinctive of the Christian faith but the early church was plagued by contrary views. Trinity Sunday spread throughout the western church around the 14th century.

In some churches, the liturgical color is white on Trinity Sunday. Images of the Trinity of the Father, Son and Holy Spirit depict that there are three elements of one God. The Trinity is sometimes symbolized by a triangular shape formed by three overlapping oval rings or loops, each featuring the one of the following images: the crown (father) within the top loop; the lamb (son) within the second loop (clockwise); and the dove (spirit) within the third loop (clockwise). Other shapes depicting the Trinity include Borromean rings and the triquetra.

The Borromean rings feature three overlapping round circles forming a triangular shape, and in the center, inside all the circles is the word "unitas". The Borromean rings are named after the crest of the Borromeo family in 15th-century Tuscany. The triquetra symbol uses shapes similar to one of the oldest Christian symbols. In the triquetra, the three equal arches of the circle express the equality of the Father, Son, and Holy Spirit. The union of the arches represents the unity of the Godhead. Their continuous form symbolizes eternity. Their interwoven appearance denotes the indivisibility of the Trinity. In the center of the triquetra is an equilateral triangle, one of the most ancient Trinity symbols, and each pair of arches formed an ellipse, the symbol of God’s glory.