Today marks the commemoration of the First Ecumenical Council:
"The First Ecumenical Council was held in Nicea in A.D. 325 and set a pattern for all later Ecumenical Councils. It primarily addressed the issue of Arianism (producing the original version of the Nicene Creed) and set a universal pattern for calculating the date of Pascha—the Paschalion. It is also referred to as the First Council of Nicea."It should be noted that even though this first council was called by St. Constantine - who was emperor at the time - it was for unity among the empire that was the driving force. St. Constantine was not trying to make Christianity in to some new kind of religion filled with paganism. By this time in Church history a few heresies had popped up, most notably Arianism. This was causing disunity in the Church and the First Ecumenical Council helped put an end to Arianism.
(Note I edited this after I was informed that I was mistaken about the dating of Pascha)
Another issue resolved during this council was the calculation for the dating of Pascha (Easter), which the Orthodox Church still uses today. This was not a date set for Pascha but rather a way for the early Christians to celebrate feasts at the same time.
This council is know as the First Council of Nicea because it was the first council to be held in the town of Nicea. The Second Council of Nicea was the Seventh Ecumenical Council - the one that defended the use of Icons in the Church and over threw iconoclasm (which will be discussed slightly in the part two of the above link).