In 325 AD, the great Council of Nicea was called by Constantine the Great, who had converted to Christianity a decade earlier and replaced official persecution of Christianity with official support. The Council of Nicea was the first ecumenical (worldwide) council of the church and the first of Seven Ecumenical Councils recognized by most Christian denominations as having doctrinal authority. Around 300 bishops from across the Christian world attended.

The main reason for the council was the dispute over Arianism, the doctrine that Christ was not equal with God but a lesser divine being, but the assembled bishops also dealt with the date of Easter and various matters of church administration. The Council was originally planned to be held in another city, but Constantine moved the location to Nicea because of its favorable weather and, most importantly, proximity to his palace in Nicomedia (modern Izmit).

Another important council was held at Nicea in 787 to deal with the iconoclastic controversy (the dispute over whether the use of icons was appropriate or constituted idolatry). This is known as the Second Council of Nicea and the Seventh Ecumenical Council. It concluded that icons were worthy of veneration but not worship, and restored their use in the Byzantine Empire.