The First Council of Nicaea was convened by Emperor Constantine the Great upon the recommendations of a synod led by Hosius of Córdoba in the Eastertide of 325. To most bishops, the teachings of Arius were heretical and dangerous to the salvation of souls. In the summer of 325, the bishops of all provinces were summoned to Nicaea, as a convenient place accessible to delegates of many countries, particularly those of Asia Minor, Georgia, Armenia, Syria, Palestine, Egypt, Greece, and Thrace.
This was the first general council in the history of the Church since the Apostolic Council of Jerusalem. The Apostolic council established the conditions upon which Gentiles could join the Church. The Council of Nicaea has to define revealed doctrine more precisely in response to a challenge from the teachings of Arius.
1800 bishops of the Christian church (about 1000 in the east and 800 in the west) were invited but a smaller number attended. Many writers mentioned the number as 318. This number 318 is preserved in the liturgies of the Eastern Orthodox Church and the Coptic Orthodox Church of Alexandria.
In the people who attended, there were some supporters of Arius including Secundus of Ptolemais, Theonus of Marmarica, Zphyrius, and Dathes, all of whom hailed from the Libyan Pentapolis, Eusebius of Nicomedia, Paulinus of Tyrus, Actius of Lydda, Menophantus of Ephesus, and Theognus of Nicaea.
The agenda included:
The Arian question regarding the relationship between God the Father and the Son (not only in his incarnate form as Jesus, but also in his nature before the creation of the world); i.e., are the Father and Son one in divine purpose only or also one in being?
The date of celebration of Pascha/Easter
The Meletian schism
Various matters of church discipline, which resulted in twenty canons
Church structures: focused on the ordering of the episcopacy
Dignity of the clergy: issues of ordination at all levels and of suitability of behavior and background for clergy
Reconciliation of the lapsed: establishing norms for public repentance and penance
Readmission to the Church of heretics and schismatics: including issues of when reordination and/or rebaptism were to be required
Liturgical practice: including the place of deacons, and the practice of standing at prayer during liturgy
The council was formally opened May 20, in the central structure of the imperial palace at Nicaea.
Eusebius of Caesarea called to mind the baptismal creed of his own diocese at Caesarea at Palestine, as a form of reconciliation. The majority of the bishops agreed. For some time, scholars thought that the original Nicene Creed was based on this statement of Eusebius. Today, most scholars think that the Creed is derived from the baptismal creed of Jerusalem, as Hans Lietzmann proposed.
The orthodox bishops won approval of every one of their proposals regarding the Creed. After being in session for an entire month, the council promulgated on June 19 the original Nicene Creed.
The Creed was amended to a new version by the First Council of Constantinople in 381.
https://www.christianhistoryinstitute.org/study/module/nicea/
http://en.wikipedia.org/wiki/First_Council_of_Nicaea
http://www.papalencyclicals.net/Councils/ecum01.htm
