The Council of Nicaea is one of the most important events in Christian history.
It is also surrounded by myths.
Why Did the Bishops Gather?
The central controversy concerned Arius and the relationship of the Son to the Father.
The council confessed the Son as truly divine and used the famous language homoousios --- of one substance/consubstantial with the Father.
What Does Nicaea Show About the Church?
Whatever later Christians think about Catholic claims, Nicaea demonstrates that fourth-century Christianity possessed:
- bishops;
- councils;
- creedal definitions;
- ecclesiastical jurisdictions;
- disciplinary canons;
- sacramental practices;
- mechanisms for addressing doctrinal crisis.
The surviving canons even discuss Eucharistic communion, bishops, ordination and reconciliation.
Primary Source
https://www.newadvent.org/fathers/3801.htm
Did Nicaea Decide the Bible?
No.
The popular claim that Nicaea selected the books of the Bible is historically inaccurate.
Read: Did the Council of Nicaea Decide the Bible? →
A Bigger Question
Christians across many traditions accept Nicene Trinitarian doctrine.
So ask:
What kind of Church possessed the authority to gather, deliberate and defend this doctrine?