Constantine's Conversion to Christianity Led to its Growth and Power

 

The following is an excerpt from a world history article here: 

In 324 CE, Constantine defeated Licinius and became the sole emperor. In that position, he essentially expanded the ideas of Aurelian, in that he could now enforce "One God, One Emperor, One Church".

The First Council of Nicaea

After mediating the Donatist Schism, his next major challenge came in 325 CE. A presbyter in Alexandria, Arius, had been teaching that at some point, God had created Christ. Riots had broken out in several cities, and Constantine brought the bishops together at the city of Nicaea to resolve the issue. The Council of Nicaea resulted in the Christian doctrine known as the Trinity, which articulated the relationship between God and Christ. The Council voted to claim that Christ was of the identical essence of God, present at creation, and manifest (incarnated) on earth in Jesus of Nazareth. Until Christ returned, the now Christian emperor stands in for Christ, and so carries the identical power of God on earth as he rules. It was after this council that Christian emperors began to be portrayed with halos over their heads, and the trappings of divine worship.

 

The concept of a creed (from the Latin credo, "I believe") was a Christian innovation. With multiple native cults, there was no central authority that dictated what all should believe. The Nicene Creed formalized one system of belief that was promoted through the power of the emperor. As such, any dissent was not only heresy but also treason.

 

The Council of Nicaea also set the date for the empire-wide celebration of Easter. Some communities had insisted on following the gospel tradition of observance during the Jewish Passover. Constantine allegedly wrote:

... 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 Savior a different way. (Eusebius, Life of Constantine, III, chapter XVII, quoted in Schaff)

Constantine elected the practice which churches in Rome followed: the first Sunday after the first full moon following the spring equinox. The later law codes under Theodosius I (r. 379-395 CE) and Justinian I (r. 527-565 CE) claimed that Constantine also created legislation against the Jews: Jews could not seek converts, were forbidden to own Christian slaves, and could not circumcise their slaves. Christians who converted to Judaism were to receive the death penalty. On the other hand, Jewish clergy were offered the same tax exemptions as Christians.

Constantine is often credited with determining the date for Christmas, too, although no edict has survived. Christians in Rome celebrated the event during the festival of Saturnalia in December 25. December was also the birthday of Sol Invictus and Mithras and may have been utilized in attempts to unify these festivals. From an ancient calendar, we know that in the year 336 CE, at least in Rome, the celebration was established on 25 December.