Arianism is the belief that Jesus is only the Son of God, and not God. All Arians are not Christian.
It was taught mainly by a presbyter called Arius of Alexandria and his followers.

Early on it was condemned by Saint Cyrill of Alexandria and other presbyters who called Arius a heretic. Even after it was condemned the heresy was still widely taught throughout the Roman Empire.
Constantine the Great gathered together around 300 elders from across the empire, mainly from the east though, and assembled them in the First Council of Nicaea.

At the First Council of Nicaea the heresy of Arianism was condemned, mainly by Saint Athanasius who cited the church fathers and scripture to prove Arius was teaching false doctrine. The most supporting verse from scripture is Matthew 28:18-20; 18 When Jesus came near, he spoke to them. He said, “All authority in heaven and on earth has been given to me. 19 So wherever you go, make disciples of all nations: Baptize them in the name of the Father, and of the Son, and of the Holy Spirit. 20 Teach them to do everything I have commanded you. As for the Church Fathers, Polycarp’s Letter to the Philippians contains a classic formulation in which he refutes the Gnostics’ argument that God’s Incarnation in, and the death and Resurrection of, Christ were all imaginary phenomena of purely moral or mythological significance.

Also. in the council they set the date of Easter and formally condemned Gnosticism. They did NOT decide the Biblical canon here, that is a myth.
The First Ecumenical Council, or The First Council of Nicaea, is where the Nicene Creed was first created, later at the Council of Constantinople it was updated to include the Holy Spirit, and even later in 589 AD to include the Filioque, which means "and the Son".
Here is the entire creed, it outlines the fundamentals of the Christian faith, although the Orthodox do disagree on the Filioque:

We believe in one God,
the Father almighty,
maker of heaven and earth,
of all things visible and invisible.

And in one Lord Jesus Christ,
the only Son of God,
begotten from the Father before all ages,
God from God,
Light from Light,
true God from true God,
begotten, not made;
of the same essence as the Father.
Through him all things were made.
For us and for our salvation
he came down from heaven;
he became incarnate by the Holy Spirit and the virgin Mary,
and was made human.
He was crucified for us under Pontius Pilate;
he suffered and was buried.
The third day he rose again, according to the Scriptures.
He ascended to heaven
and is seated at the right hand of the Father.
He will come again with glory
to judge the living and the dead.
His kingdom will never end.

And we believe in the Holy Spirit,
the Lord, the giver of life.
He proceeds from the Father and the Son,
and with the Father and the Son is worshiped and glorified.
He spoke through the prophets.
We believe in one holy catholic and apostolic church.
We affirm one baptism for the forgiveness of sins.
We look forward to the resurrection of the dead,
and to life in the world to come. Amen.

Belief in Arianism is against the fundamentals of Christian faith; it is a Unitarian doctrine and should be treated as such. Some groups and teachers of Arianism are the Jehovah's Witnesses and their leaders.