The Ecumenical Councils
Among Christians, there sometimes appear people who incorrectly expound on Christian teachings and who want their incorrect teaching to be binding on the whole Church.
The Church calls such false teaching heresy and the false teachers — heretics. When general persecution of the Christians ceased, heresies’ especially started to trouble the life of the Church. The Devil, unable to defeat the Church from without, began to attack it from within.
Following the example of the Apostles, councils of bishops, pastors, and teachers of the Church from the whole known world were convened for the struggle against the heretics and for the instruction of all Christians in the correct Orthodox faith
When it was necessary to resolve the question whether Christians converted from pagan religions should fulfil the ceremonies of the laws of Moses, the Holy Fathers gathered at the Apostolic Council of 51 A.D. in Jerusalem (see Acts 15:1-35). So also, the fathers of the Church, the bishops to whom the apostles transferred their power received by laying on of hands, gathered at the Councils when some sort of teaching contrary to Christian faith arose.
These general conferences of the entire Christian world, called Ecumenical Councils, maintained the truth of Christ by the guidance of the Holy Spirit and participation of the whole Christian Church. There have been seven Ecumenical Councils.
At the First and Second Ecumenical Councils, the Symbol of Faith or Creed was composed. The Creed is a short statement of all the tenets of the Orthodox Christian faith. Every Orthodox Christian is obliged to know it. We sing it at the Divine Liturgy and recite it during our morning prayers.
The Seventh Ecumenical Council defended and affirmed the veneration of holy icons.
(from: The Law of God by Fr. Seraphim Slobodskoy)