The Sunday following Pentecost Sunday is dedicated not to an event like Christmas Day or Easter, but to the teaching of the Bible concerning the identity of God. Mankind has always known that God exists by the things He created and by the conscience. Yet God desires all people to know who He is, and so He revealed many things about Himself in the Bible.
In His own words, He speaks about Himself. Unlike the mythological gods, created by men in their own image, the God who reveals Himself in the Bible is fundamentally different from man. He tells us that He is the one and only God, and He tells us that He is three Person: Father, Son and Holy Spirit. The Father is the only true God. The Son is the only true God, and the Holy Spirit is the only true God. The Father is neither the Son nor the Holy Spirit. The Son is neither the Father nor the Holy Spirit, and likewise the Holy Spirit is neither the Father nor the Son. Thus God is three-in-one, and so He is often called the Holy Trinity by Christians the world over.
How this all can be is not something that is accessible to human reason. That each human being can fit all the world that he or she knows between the ears is miraculous, but God is not finite and far beyond anything that can be imagined, so Trinity Sunday is an expression of trust in God’s Word, that we believe what God says about Himself by faith.
In many churches, including Our Saviour, the Athanasian Creed is recited. A creed is simply an expression of what a person believes (The Latin word credo simply means “I believe”.) The Athanasian Creed was named for Athanasius, an early Christian who clearly confessed God’s three-in-one nature. While it is the longest of the three creeds that are commonly used by Christians to confess their beliefs together, it is the one that most clearly spells out the teaching of the Bible that the one true God is three Persons in one divine Being.