Arianism asserts that Jesus Christ is the Son of God who was created by God the Father at a point in time, is distinct from the Father and is therefore subordinate to the Father.
Arius (c. AD 250–336), a Christian presbyter in Alexandria, Egypt, is most generally thought of as the first proponent of the Heresy that bears his name.
In 325 Constantine was instrumental (pivotal) in calling the 1st Nicene Council principally to address the question of Arianism. The Nicene Council found against Arius who was sent into exile with his followers, though some years later allowed to return, and Constantine was baptised by Eusebius (an Arian Bishop) around 336.
The Cappadocian Fathers were very strong in their rejection of Arian teaching, and at the 1st Council of Constantinople 381 the new and revised Nicene Creed was approved with much a much stronger Christology designed specifically to exclude Arian teach from the Church.
In 390 Ambrose of Milan – who was a Nicene Christian but keen to keep the Arians of his Diocese on side wrote to the Pope advocating the Apostles Creed as a sufficient statement.
The principal problem of Arianism is that it destroys the Christology of the Fourth Gospel, and most especially as demonstrated in the first 18 verses of the Gospel.
John 1:1 In the beginning was the word and the word was with God and the word was God.