The Council of Nicaea was called by the church at the direction of the Emperor Constantine, primarily to decide the major theological controversies of the early 4th century. As a result, the Nicene Creed was issued by the Council, and the doctrines of the Nestorians and the Arians were condemned. Their rejection of the Trinity was condemned, and those who upheld this rejection it were ordered to be executed. The Nestorians and Arians held that Jesus Christ was not God, but only a man, and therefore rejected the second person of the Trinity.
To this day, most Christian denominations require adherence to the Nicene Creed for their members. A general term for Christians who accept the Creed is 'Trinitarians'. Those who reject it are known as 'Unitarians'.
Read about it in the works of the Ecumenical Patriarch, St. Photios the Great. The addition of filoque by Rome, was one of the causes of the Great Schism.
It still separates the Eastern Orthodox Christians from BOTH Rome and the Prostestants.