At the time Arius from Alexandria challenged Constantine's view that Jesus is God. This result a split in the church which were referred as Arianism.

Therefore, the church leaders gathered in 325 AD at the council of Nicaea to prevent further church splits by:

• defining the divinity of Jesus;
• condemn Arianism as heretical;
• establishing church hierarchy;

• separating Christian practices from Jewish traditions;

  The Emperor Constantine actively participated in the proceedings, as well as 300 bishops and church leaders from across the Roman Empire attended and established the Nicene Creed (statement of faith) which is:

• We believe in one God, the father almighty, maker of heaven and earth and of all things visible and invisible.
• And in one lord, Jesus the anointed,
  • the only begotten son of God,
  • begotten of the father before all worlds,
  • light from light,
  • true God from true God,
  • begotten not made, being of one substance (homousion) with the father (against Ebionitism and Arianism),
  • by whom all things were made.
  • Who for us humans and for our salvation came down from heaven and was incarnate by the holy spirit and the virgin Mary (against Docetism), and was made man, and
  • was crucified also for us under Pontius Pilate.
  • He suffered and was buried, and the third day he rose again according to the scriptures, and ascended into heaven, and sits at the right hand of the father.
  • And he shall come again to judge both the living and the dead.
  • Whose kingdom shall have no end.

Eusebius initially showed sympathy for Arius, but eventually decided to sign the Nicene Creed which had the following outcome:

• Defined the Holy Trinity
• Declared Jesus Christ as divine
• Declared Jesus Christ was the same substance ("homoousios") as God the Father
• Declared anyone who believed Jesus was a created being a heretic (meaning they may be prosecuted because they stir division in their church)
• Establish the authority and jurisdictions of bishops within the Church hierarchy
• Christians should separate themselves from Jewish feasts (according to them Jesus Christ "fulfilled" the Jewish feats)
• Jewish passover had to be replaced with the Christian Easter (to establish unity, because this feast was also celebrated by the pagans)

This creed become the cornerstone of the Christian belief and continues to be recited in many modern Christian denominations during worship services.

In 336 Arius died, potentially poisoned by his opponents, others claim it was the result of "God's punishment for his heretical views" (against the Trinity doctrine).

The first Christmas

On 25 December 336 AD the first recorded instance of Christmas was celebrated by Christians, the day of the Saturnalia feast which is also the day of the celebration of the birth of the sun deity Sol Invictus. This happens to be also the day of the turning of Winter in the Northern Hemisphere, when the length of the days (time of sunlight) starts to increase daily.

The first Easter

Constantine supported the separation of the date of Easter from the Jewish Passover, stating in his letter after the First Council of Nicaea:

"... it appeared an unworthy thing that in the celebration of this most holy feast we should follow the practice of the Jews, who have impiously defiled their hands with enormous sin, and are, therefore, deservedly afflicted with blindness of soul ... Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Saviour a different way." -- Eusebius, Life of Constantine Vol. III Ch. XVIII Life of Constantine (Book III)