A lot of Christians believe and will say that unless you believe the Trinity doctrine, of one God in three, coequal, coeternal, persons you can't be a Christian. Why do they say that? It's not in the Bible. Here's why they say it.
Athanasian Creed, approx. 450 AD.
Whoever wants to be saved should above all cling to the catholic faith.
Whoever does not guard it whole and inviolable will doubtless perish eternally.
Now this is the catholic faith: We worship one God in trinity and the Trinity in unity, neither confusing the persons nor dividing the divine being.
For the Father is one person, the Son is another, and the Spirit is still another.
But the deity of the Father, Son, and Holy Spirit is one, equal in glory, coeternal in majesty.
What the Father is, the Son is, and so is the Holy Spirit.
Uncreated is the Father; uncreated is the Son; uncreated is the Spirit.
The Father is infinite; the Son is infinite; the Holy Spirit is infinite.
Eternal is the Father; eternal is the Son; eternal is the Spirit: And yet there are not three eternal beings, but one who is eternal; as there are not three uncreated and unlimited beings, but one who is uncreated and unlimited.
Almighty is the Father; almighty is the Son; almighty is the Spirit: And yet there are not three almighty beings, but one who is almighty.
Thus the Father is God; the Son is God; the Holy Spirit is God: And yet there are not three gods, but one God.
Thus the Father is Lord; the Son is Lord; the Holy Spirit is Lord: And yet there are not three lords, but one Lord.
As Christian truth compels us to acknowledge each distinct person as God and Lord, so catholic religion forbids us to say that there are three gods or lords.
The Father was neither made nor created nor begotten; the Son was neither made nor created, but was alone begotten of the Father; the Spirit was neither made nor created, but is proceeding from the Father and the Son.
Thus there is one Father, not three fathers; one Son, not three sons; one Holy Spirit, not three spirits.
And in this Trinity, no one is before or after, greater or less than the other; but all three persons are in themselves, coeternal and coequal; and so we must worship the Trinity in unity and the one God in three persons.
Whoever wants to be saved should think thus about the Trinity.
It is necessary for eternal salvation that one also faithfully believe that our Lord Jesus Christ became flesh.
For this is the true faith that we believe and confess: That our Lord Jesus Christ, God’s Son, is both God and man.
He is God, begotten before all worlds from the being of the Father, and he is man, born in the world from the being of his mother — existing fully as God, and fully as man with a rational soul and a human body; equal to the Father in divinity, subordinate to the Father in humanity.
Although he is God and man, he is not divided, but is one Christ.
He is united because God has taken humanity into himself; he does not transform deity into humanity.
He is completely one in the unity of his person, without confusing his natures.
For as the rational soul and body are one person, so the one Christ is God and man.
He suffered death for our salvation. He descended into hell and rose again from the dead.
He ascended into heaven and is seated at the right hand of the Father.
He will come again to judge the living and the dead.
It
Those who have done good will enter eternal life, those who have done evil will enter eternal fire.
This is the catholic faith.
One cannot be saved without believing this firmly and faithfully.
Anyone who reads this will see it's logically contradictory. Three cannot be one. It says the Father, Son, and Holy Sprit are all eternal but they are not three eternal beings, but one. That's a logical contradiction. That's why no one can explain it. So, how could they get people to believe this? Keep the Scriptures from them and tell them if you don't believe this you're going to burn in hell for eternity. It's not hard to see how those uneducated masses came to accept this. It became church dogma in the Catholic church and has been preached ever since. There's nothing in the Bible that say one has to believe in the Trinity to be saved. The Bible says one must believe that Jesus is the Christ to be saved.
What was believed before that? The Nicene Creed.
THE NICENE CREED (A.D. 325)
We believe in one God, the Father, Ruler of All,
Maker of all things visible and invisible;
And we believe in one Lord Jesus Christ, the Son of God,
Begotten of the Father as only begotten,
That is, from the essence of the Father,
God from God, Light from Light, true God from true God,
Begotten, not created, of the same essence as the Father,
Through whom all things came into being, both in heaven and in earth;
Who for us and for our salvation came down,
And was incarnate, being made a man.
He suffered and the third day he rose.
And he ascended into the heavens.
And he will come to judge the living and the dead.
And we believe in the Holy Spirit. Amen.
This is the first church creed concerning the relation of the Father and the Son. Notice they believed what Paul believed. They believed in one God, the Father. Notice too that they believed that Jesus was God from God. Literally God out of God. We find Jesus said that same thing.
Jesus said unto them, If God were your Father, ye would love me: for I proceeded forth and came from God; neither came I of myself, but he sent me. (Jn. 8:42 KJV)
Literally, came out of God.
We can see how their ideas changed over a short period of time. If you look at the 381 AD. revision of the Nicene Creed you can see how they moved a little closer to the Athanasian Creed.