Trinity Sunday is the first
Sunday after Pentecost in the Western Christian liturgical calendar, and the Sunday of Pentecost in Eastern Christianity. Trinity Sunday celebrates the Christian doctrine of the Trinity, the three Persons of God: the Father, the Son, and the Holy Spirit.
The Athanasian Creed is attributed to Athanasius, the fourth century bishop of Alexandria who was the strongest defender of the doctrines of the Trinity and the divinity of Christ. It defines the doctrines of the Trinity and the nature of Christ in very concise language.
Note that the term "catholic" with the lower case 'c' is not a reference to the Roman Catholic Church, but is a reference to the universal Christian faith, since that is how the term was originally used.
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Athanasian Creed
Whoever desires to be saved should above all hold to the catholic faith.
Anyone who does not keep it whole and unbroken will doubtless perish eternally.
Now this is the catholic faith:
That we worship one God in trinity and the trinity in unity,
neither blending their persons
nor dividing their essence.
For the person of the Father is a distinct person,
the person of the Son is another,
and that of the Holy Spirit still another.
But the divinity of the Father, Son, and Holy Spirit is one,
their glory equal, their majesty coeternal.
