Did the Council of Nicaea Decide the Bible?

No. The First Council of Nicaea in AD 325 did not decide which books belong in the Bible.

Nicaea was principally concerned with the controversy over Arius and the identity of Jesus Christ, along with matters of Church discipline. Its surviving canons concern ecclesiastical questions; they do not contain a decree selecting the 27 books of the New Testament.

That makes the popular story that bishops met at Nicaea and "voted on the Bible" historically misleading.

What did Nicaea actually decide?

The council defended the full divinity of the Son against Arian teaching and produced the creed associated with Nicaea. Its disciplinary canons address matters such as clergy, bishops, jurisdiction and Church order.

The surviving text can be read directly:

https://www.newadvent.org/fathers/3801.htm

So when was the New Testament canon recognized?

Recognition developed over time.

Many New Testament books were received very early and broadly. Some books remained disputed in parts of the Christian world.

An important milestone is Athanasius' Festal Letter 39 in AD 367, which lists the same 27 New Testament books Christians use today.

https://www.newadvent.org/fathers/2806039.htm

Regional councils in North Africa later issued canonical lists for ecclesial use.

The Catholic claim is not that a council made inspired books inspired. It is that the Church discerned which writings belonged to the canon.

Why does the myth matter?

Because it distracts from the more interesting historical question:

How did Christians know which writings were Apostolic Scripture before there was a universally settled New Testament table of contents?

That question leads directly into the history of Church authority, Apostolic Tradition and the canon.

Go deeper

Who Decided Which Books Belong in the Bible? →

Scripture and Tradition in the Early Church →

Who Has Authority to Interpret the Bible? →