The Nicene Creed (325 C.E.)

Within 300 years, the number of Christians had grown from a handful to make up about half the Roman Empire's population. When he assumed his place as emperor of the western Roman empire, Constantine pressed for the Edict of Milan (313 C.E.), ending the threat of the persecution of Christians at the hands of the Roman government. By 325, Constantine had assumed the position of emperor over the eastern Roman empire as well, and needed a unifying religion to end the divisions the empire had been experiencing. Although he himself was not a Christian, he established Christianity as the empire's official religion, undoubtedly because it included so much of the empire already, and was always increasing its numbers.

But the official religion was not a single entity. It was divided between Apostolic and Gnostic traditions, and even within the Apostolic tradition there were fierce disagreements about the person of Jesus.

Constantine commanded all the bishops of the Church to assemble at Nicea, just outside of Constantinople (modern Ankara). This move effectively ousted the Gnostic Christians from the game (they had no hierarchical structure, thus no bishops). The Apostolic bishops were told by the presiding (pagan) Constantine that they must establish agreement -- any agreement.

The result was the Nicene Creed.

 
We believe in one God,
the Father, the Almighty,
maker of heaven and earth,
of all that is, seen and unseen

We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, Light from Light,
true God from true God,
begotten, not made,
of one Being with the Father.
Through him all things were made.
For us and for our salvation
he came down from heaven;
by the power of the Holy Spirit
he became incarnate from the virgin Mary, and was made man
For our sake he was crucified under Pontius Pilate;
he suffered death and was buried.
On the third day he rose again
in accordance with the Scriptures;
he ascended into heaven
and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead,
and his kingdom will have no end.

We believe in the Holy Spirit, [the Lord, the giver of life,
who proceeds from the Father and the Son]*
With the Father and the Son he is worshiped and glorified.
He has spoken through the prophets.
We believe in one holy catholic and apostolic Church.
We acknowledge one Baptism for the forgiveness of sin.
We look for the resurrection of the dead,
and the life of the world to come. Amen

The words "the Lord, the Giver of life, who proceeds from the Father" were added in 381, and the words "and the Son" were added in the Western [Catholic] Church in 589. The Eastern [Orthodox] Church declared the phrase "and the Son" heretical in 864.