| The Ecumenical council is a gathering of bishops to determine weightly religious matters. There have been seven such councils:
The Great & Holy Council of Nicea
The 2nd Great & Holy Council - Constantinople, 381 AD
The 3rd Great & Holy Council - Ephesus, 431
The 4th Great & Holy Council - Chalcedon, 451
The 5th Great & Holy Council - The 2nd Council of Constantinople, 553
The 6th Great & Holy Council - The 3rd Council of Constantinople, 680
The 7th Great & Holy Council - Nicea, Asia Minor, 787 A.D. - Affirmed the propriety of icons as genuine expressions of the Christian Faith.
The Nicean council is famous for making the Trinity official Christian doctrine and declaring competitive doctrines as heresies.
Most of this information was taken from goholycross.org | 
