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.
The Nicene definition of the Trinity developed over time, based on Scripture and Tradition. (Here is the evolution of the creed). The Scriptures call the Father, the Son, and the Holy Spirit "God," yet the three are also clearly distinct. For instance, John gives Jesus the titles theos and monogenes theos (God and Only-Begotten God) and has Jesus saying that the Father and Son are one, yet in his gospel Jesus also states that the Father and Son are not one witness, but two (John 1:1, 18; 8:17-18; 10:30). So John tells us that Jesus is God but not God the Father? Jesus is one with the Father, but they constitute two witnesses?
It is scriptures such as these that led to the development of the Trinity doctrine. The Church had to reconcile the Divinity of Christ and the Holy Spirit with Jewish monotheism. Over time, and with the aid of the Holy Spirit, the Church reflected on the implications of God’s nature, and even began using the word Trinity by the middle of the 2nd century to describe the relationship between the Father, Son, and Spirit.
The immediate political need for the church to resolve conflict between opposing views, and to contribute to the social cohesion of the late Roman Empire, was also a powerful influence on the process and its outcomes. When in the 4th century a presbyter named Arius denied the Father and Son were both true God and co-eternal, his bishop Alexander of Alexandria challenged him and deposed him.
Arius taught Jesus was a creature – the first creation of God, necessary to mediate between the unknowable distant God (a concept borrowed from Platonic thought) and creation. It is this belief that led to the phrase "very God or very God" describing Jesus in the Nicene Creed to counter Arius.
Eventually the Arian controversy spread, and the emperor Constantine, newly fascinated with Christianity, convened a council of bishops in AD 325 in Nicaea to deal with Arianism. It is there that the Church drew up the beginnings of the current Nicene Creed. In the latter half of the 4th century the Church dealt with those who specifically denied the divinity of the Holy Spirit, adding more text to the creed.
Ultimately, the trinity depicts a dynamic God, whose ultimate nature is beyond human conception, yet who voluntarily operates within the created world. Trinitarianism also shows a loving God that is willing to become as we are so that we may become like Him.
The implications of believing in Arius’ God, a God unwilling to involve himself in our redemption, but who instead sent an angel of the highest order, did not escape the earliest Christians. As St. Athanasius was fond of saying "that which has not been assumed has not been redeemed," meaning that unless God truly became completely human, we could not be fully redeemed, because only God Himself is capable of truly redeeming humanity; an angel does not have this ability. Thus, the trinity is about the heart of our salvation.