The first Ecumenical Council, which was held in Nicaea of Bithynia, was concluded on June 19th 325 AD. In this Council for the first time fundamental articles of faith were codified, which, with various articles of faith, which, with various modifications by the Council of Chalkedon in 451, constituted the “Nicene Creed”, the core beliefs of Christianity. At the same time, at the Council of Nicaea it was decided that Easter will be celebrated in the same day with the rest of the Christian world, the first Sunday after the full moon that coincides or follows the spring equinox.