The Nicene Council, also known as First Ecumenical Council of the Catholic 
Church, was held in 325 and is perhaps one of the most important councils in 
Christian history. It was originally called by Emperor Constantine in order to 
address the challenges posed by Arianism. The council established the 
foundations of orthodox Christian belief with the Nicene Creed: 

We believe (I believe) in one God, the Father Almighty, maker of heaven and 
earth, and of all things visible and invisible. And in one Lord Jesus Christ, 
the only begotten Son of God, and born of the Father before all ages. (God of 
God) light of light, true God of true God.

Begotten not made, consubstantial to the Father, by whom all things were made. 
Who for us men and for our salvation came down from heaven.

And was incarnate of the Holy Ghost and of the Virgin Mary and was made man; was 
crucified also for us under Pontius Pilate, suffered and was buried; and the 
third day rose again according to the Scriptures.

And ascended into heaven, sits at the right hand of the Father, and shall come 
again with glory to judge the living and the dead, of whose Kingdom there shall 
be no end.

And (I believe) in the Holy Ghost, the Lord and Giver of life, who proceeds from 
the Father (and the Son), who together with the Father and the Son is to be 
adored and glorified, who spoke by the Prophets.

And one holy, catholic, and apostolic Church. We confess (I confess) one baptism 
for the remission of sins.

And we look for (I look for) the resurrection of the dead and the life of the 
world to come. Amen." 

In 381 the Nicene Creed was altered slightly: 

We believe in one God,
the Father, the Almighty,
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 worshipped 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 sins.

We look for the resurrection of the dead,
and the life of the world to come. Amen.