Council of Nicea
There where two Councils of Nicaea which where the first and seventh of Ecumenical councils.
When dealing with born again Christians they are often completely ignorant of these Councils and that what they believe was decided at these councils. Often what was decided at one would be overturned at another. The honoring of icons is a good example of this.
Arguably the most significant aspect of Christianity decided at a council was the Trinity.