The First Ecumenical Council, also known as the First Council of Nicaea, was convened in 325 AD in the city of Nicaea (present-day İznik, Turkey) by the Roman Emperor Constantine I. This council is widely regarded as a pivotal moment in the history of Christianity, as it was the first gathering to unite bishops from across the Christian world to deliberate on theology and church practice in an effort to foster unity and resolve major doctrinal disputes (Britannica)[https://www.britannica.com/event/First-Council-of-Nicaea-325], (Wikipedia)[https://en.wikipedia.org/wiki/First_Council_of_Nicaea].
The central issue addressed at the council was the Arian controversy. Arius, a priest from Alexandria, taught that Jesus Christ was not divine in the same way as God the Father, undermining the doctrine of the Trinity. This caused significant division within the Church. The council was primarily convened to address this heresy and to define the true nature of Christ (Britannica)[https://www.britannica.com/event/First-Council-of-Nicaea-325], (Greek Orthodox Archdiocese)[https://www.goarch.org/-/the-importance-of-the-council-of-nicaea-325-ad-].
The council was attended by roughly 300 bishops—some sources say 318—along with other church officials and representatives. These bishops came from across the Roman Empire, including regions such as Hispania, Italiciana, Asia, and Pontus. Notable participants included St. Alexander of Alexandria, Eustathius of Antioch, and Hosius of Cordova, who played a significant advisory role to Constantine. Pope Sylvester I did not attend in person due to age but was represented by his legates (New Advent)[https://www.newadvent.org/cathen/11044a.htm], (OrthodoxWiki)[https://orthodoxwiki.org/First_Ecumenical_Council].
The most significant result of the council was the formulation of the original Nicene Creed, a statement of Christian faith which clearly declared that the Son (Jesus Christ) was “of one substance” (homoousios) with God the Father, affirming the full divinity of Christ and countering Arianism. Delegates also decided on a uniform method for determining the date of Easter, independent of the Jewish calendar, and the council resolved several administrative and disciplinary questions, setting a precedent for future ecumenical councils (Wikipedia)[https://en.wikipedia.org/wiki/First_Council_of_Nicaea], (Britannica)[https://www.britannica.com/event/First-Council-of-Nicaea-325].
The council’s decisions—especially the acceptance of the Nicene Creed—became foundational for the doctrine of the Trinity and ecclesiastical unity within Christianity. The Council of Nicaea also demonstrated the growing influence of imperial authority over church affairs and set the pattern for future ecumenical councils (Britannica)[https://www.britannica.com/event/First-Council-of-Nicaea-325].
In summary, the First Ecumenical Council at Nicaea in 325 AD was a seminal event for Christianity, laying the groundwork for essential doctrines and organizational principles that continue to shape Christian communities today.