The Council of Nicea in 325CE following the Roman Empire’s Edict of Toleration of 313CE was one of the first and most important of the councils ever held by the Church presided over by the Emperor Constantine. The decisions of the council set the trajectory of the Church to its eventual split into East and West, and marked the arrival of the Papacy into its organisation. It also dealt with some heresies and formulated an Emperor friendly theology (Eusebianism). For some it marked a catastrophic failure of the Church as it merged with the Empire to become a state Church, losing its distinctive witness to the world. The essay was limited to 1500 words but does a good job at introducing the important themes of the Council.
A PDF is found here.