The church was at a crossroads. What were they going to do with God, Jesus and the Holy Spirit? The church had come to sense that in Jesus, God was fully present. In the same way the church understood that God was present in the Holy Spirit and that the Holy Spirit was a personal agent that empowered Jesus. These three were united in scripture in a way which could not be ignored (Matthew 28: 19 "Go therefore and baptize in the name of the Father, and the Son and the Holy Spirit; 2 Corinthians 13:13 "The grace of the Lord Jesus Christ, the love of God and the fellowship of the Holy Spirit be with you all."). The problem with these beliefs (that God was fully present in Jesus and the Spirit) was that the church also professed that God was one; a singular entity. So how was the church going to keep God one and yet understand God as fully present in both Jesus and the Spirit?
The end result was what we have come to know as the doctrine of the Trinity. Perhaps the easiest way to understand the Trinity (if there is such a thing) is the summary offered to us by St. Augustine. Augustine (354-430 CE) used three statements to help explain the Trinity. Here they are:
This way of speaking about God was not new to Augustine. Much earlier Theophilus of Antioch (c. 170s CE) and Tertullian (c. 210s CE) both wrote clearly about this relationship in which God was one and yet there was God the Father, God the Son (Logos) and God the Spirit (Sophia). Their purpose for writing was to defend against the beliefs that either there were multiple gods (God, Jesus, Spirit) or that Jesus and the Spirit were not really God but simply created entities through which God worked.
These discussions about God, Jesus and the Spirit came to a head in the 4th century when a bishop name Arius made it clear that in this universe there are only two real substances, God and what God created. There is no in-between. Therefore Jesus and the Spirit cannot be God and thus must be creations. This proclamation split the church. Some argued for the unity of God as the Father, Son and Holy Spirit while others sided with Arius that Jesus and the Spirit were lesser created entities. If the church was to stay united it had to develop a formula, or doctrine, which would settle the debate in a manner which was consistent with the scriptural references that spoke of the Word (Jesus) and the Spirit as being eternal and one with God, yet also referred to God as one.
As mentioned above the church settled on the doctrine of the Trinity. What is interesting in all of this is that nowhere in the Creeds is the word "Trinity" actually used to describe the relationship of God the Father, Son and Holy Spirit. In fact the word "three" is not used either. The word Trinity (first coined by Theophilus of Antioch) was simply the most easily accessible way to speak of this relationship. Two reasons it was not used in the official Creeds was first because it is not a Biblical word and second because it seemed to focus on the separation of God (into a tri-Godhead) rather than the unity of God.
Why does the Trinity matter? It matters because it reminds us that in Jesus we have seen the fullness of the very love God. It matters because it reminds us that in the presence of the Holy Spirit we are encountering God's own self. It matters because it reminds us that God became incarnate among us in Jesus and continues to be present among us through the Spirit. In many ways the Trinity tries to explain a mystery which is unexplainable…yet at the same time affirm truths to which we need to hold.