Okay I gotta say Whoa here people there is a lot of misinformation here that has taken this topic off the OP, and shows some modern misunderstanding about the history of the Trinitarian Doctrine.
1) The word "person" or "persons" is not in the The Creed of Nicea written about 325AD. Nor do the words appear in the Nicene - Constaninopolitan creed (commonly known as the Nicene Creed used in modern liturgical churches). So why you all are off on the persona think is beyond me.
2) In answer to the original OP. By the 300 AD Christianity had developed to have many different theologies on the divinity and humanity of Christ - from the Gnostics to the Arians and Modalists, just to mention very few. Constantine tasked the bishops to get together and settle the issue. So they did. Basically here is the conclusions.
- The Bible mentions God in the form of "The Father" (as referred to by Jesus), "The Son", and the "Holy Spirit" as mentioned in the New Testament writings. In the New Testament there only one place in the Bible that reference to all three is given at the same time. (more on that later). There are verses declaring the Father and Son are one, or the Son and Spirit are one, or the Father and Spirit are one. But no verse in the Bible declaring all three are one. (I'm a Trinitarian Christian so don't get mad at me for that statement, it just is the way it is.)
- The council of the Bishops (The Lateran Councils) stated it can be shown that all three are one by what these verses say.
- There is one God - so the Father, Son and Holy Spirit are three ways that God has revealed the divine to us.
- When one part of the Trinity takes action the other two are also acting in that action. (I know kind of a weird way to say it). What it means is when God created the cosmos the Son was there creating and also the Holy Spirit. Genesis 1:2b "And the Spirit of God moved upon the face of the waters." (KJV) John 1:1-4 "In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came into being through him, and without him not one thing came into being. What has come into being in him was life, and the life was the light of all people." As these two verses illustrate all three parts of the Trinity were in action together.
- The Father, Son, and Holy Spirit are in a relationship together so the will of one is the will of the others.
- How is it 3 in 1? That is the mystery we don't understand it but this is how God has been revealed to humanity so we need to be faithful to it.
3) Trinitarian Doctrine was not completely accepted but it did decide what was considered Orthodox doctrine (teaching) for Christianity.
4) So one can be a Christian - Christ follower. They can even not accept the Trinity and still be a Christian and saved for that matter as it is God that saves us, there is nothing we can do to be righteous as that is done by Christ and Christ alone. As the saying goes, "There is nothing you can do, or fail to do, to cause God to stop loving you."
- So for example, Mormons are considered Christians - they do follow Christ's commands. However they do not accept the Trinity. There are other Christian denominations, for example the Apostalic Church and Jesus Alone teachings that do not accept the Trinitarian doctrine. Because of this they are not considered mainstream or Orthodox Christians. But by definition they are Christians - Christ Followers.
5) On a couple of side notes:
- Why do we not use terms like Mother or Creator/Redeemer/Sustainer as the title for the parts of the Trinity?
- As for descriptions as the Creator/Redeemer/Sustainer they are used in some of the church liturgy and some would like to change to using this description. However these title denote tasks or jobs and imply they each do a seperate job or tasks. However as I described above the three are always in action together they all three Created, all three Redeem, and all three Sustain us.
- Why not use the term Mother? Well I am with the group that actually would not have a problem with this, as there are verses in the Bible, especially in the Psalms and Prophes, that refer to the feminine side of God. I also know women who are strong Christians and have trouble relating to God as a father because of sexual and physical abuse from their biological fathers. With that said, it is through Jesus that the trinity is first revealed. He used the term "Father" (actually the Greek word he used is more properly translated as "Daddy"). Because Jesus used the term Father we keep it to be faithful to Jesus' teaching.
Wow, that was a lot. But I think needed to be covered. My credentials on this is I went to seminary and have a Masters in Divinity. I had a very tough Doctrine teacher and he demanded that we understood the Trinitarian doctrine as really most denominations (especially my own the United Methodist Church) and Christians are really weak on it.
I'm sure I have not covered everything. But I hope this answers your question Jane_Doe, and at the same time helps to clear up some of the misconceptions on the Trinitarian doctrine that has been given in this thread.
Finally, I need to ask. Who said if you do not accept the Trinity you will go to hell? I'm not even sure this is a teaching of the Greek Orthodox who are the strongest Trinitarian Christians there are.