The First Council of Nicea is believed to have been the first Ecumenical council of the Christian Church. Most significantly, it resulted in the first uniform Christian doctrine, called the Nicene Creed. With the creation of the creed, a precedent was established for subsequent general (ecumenical) councils of Bishops' (Synods) to create statements of belief and canons of doctrinal orthodoxy— the intent being to define unity of beliefs for the whole of Christendom.
Derived from Greek oikoumenikos, "ecumenical" literally means worldwide but generally is assumed to be limited to the Roman Empire, as in Augustus' claim to be ruler of the oikoumene/world; the earliest extant uses of the term for a council are Eusebius' Life of Constantine 3.6 around 338 "σύνοδον οκουμενικν συνεκρότει" (he convoked an Ecumenical council), Athanasius' Ad Afros Epistola Synodica in 369, and the Letter in 382 to Pope Damasus I and the Latin bishops from the First Council of Constantinople.
The purpose of the council was to resolve disagreements arising from within the Church of Alexandria over the nature of Jesus in relationship to the Father; in particular, whether Jesus was the literal son of God or was he a figurative son, like the other "sons of God" in the Bible. St. Alexander of Alexandria and Athanasius took the first position; the popular presbyter Arius, from whom the term Arian controversy comes, took the second. The council decided against the Arians overwhelmingly (of the estimated 250–318 attendees, all but two voted against Arius).
Another result of the council was an agreement on when to celebrate the Resurrection, the most important feast of the ecclesiastical calendar. The council decided in favour of celebrating the resurrection on the first Sunday after the first full moon following the vernal equinox, independently of the Hebrew Calendar. It authorized the Bishop of Alexandria (presumably using the Alexandrian calendar) to announce annually the exact date to his fellow bishops.
Historically significant as the first effort to attain consensus in the church through an assembly representing all of Christendom, the Council was the first occasion for the development of technical Christology. Through it a precedent was set for subsequent general councils to adopt creeds and canons. This council is generally considered the beginning of the period of the First seven Ecumenical Councils in the History of Christianity.
Text: courtesy of wikipedia.com
(Please see my posts on October 28, 2008 and November 1, 2008 for my perspective on the Council of Nicaea).