Trinity Sunday, the first Sunday after Pentecost, honors the Holy Trinity—the Father, Son and Holy Spirit. Although the word “trinity” does not appear in Scripture, it is taught in Matthew 28:18-20 and 2 Corinthians 13:14 (and many other biblical passages). It lasts only one day, which is symbolic of the unity of the Trinity.
Trinity Sunday is one of the few feasts of the Christian Year that celebrates a reality and doctrine rather than an event or person. The Eastern Churches have no tradition of Trinity Sunday, arguing that they celebrate the Trinity every Sunday. The Western Churches did not celebrate it under the 14th century under an edict of John XXII
Since that time Western Christians have observed the Sunday after Pentecost as a time to pause and reflect on the Christian understanding of God
The intention of the creeds was to affirm the following core beliefs:
-the essential unity of God
-the complete humanity and essential divinity of Jesus
-the essential divinity of the Spirit
Understanding of all scriptural doctrine is by faith which comes through the work of the Holy Spirit; therefore, it is appropriate that this mystery is celebrated the first Sunday after the Pentecost, when the outpouring of the Holy Spirit first occurred.
The Trinity is best described in the Niceno-Constantinopolitan Creed, commonly called the Nicene Creed from 325AD.
Essentially the Trinity is the belief that God is one in essence (Greek "ousia"), but distinct in person (Greek "hypostasis"). The Greek word for person means "that which stands on its own," or "individual reality," but does not mean the persons of the Trinity are three human persons. Therefore we believe that the Father, the Son, and the Holy Spirit are somehow distinct from one another (not divided though), yet completely united in will and essence.
The Son is said to be eternally begotten of the Father, while the Holy Spirit is said to proceed from the Father through the Son. Each member of the Trinity interpenetrates one another, and each has distinct roles in creation and redemption, which is called the Divine economy. For instance, God the Father created the world through the Son and the Holy Spirit hovered over the waters at creation.