The first seven ecumenical councils that occurred between 325 and 787 were attempts by the Orthodox Church to reach a consensus, restore peace among the churches, and develop a unified Christianity by refuting popular heresies and solidifying official teachings. These seven are generally accepted by Eastern Orthodox, Roman Catholics, and Anglicans, but only partly by Oriental Orthodox and protestants. The Symbol of Faith (Nicene Creed) which we recite every Sunday was one of the primary products of the First Council of Nicea, and contains within it everything a Christian must believe, while at the same time refuting then popular heresies about the nature of the relationship between God the Father and Jesus Christ.
It can be a little confusing, because we often hear from the Fathers and Hierarchs about the dangers of ecumenism, and we may wonder how to pull this apart from these councils being called ecumenical. The word itself comes from the Greek oikoumene, which refers to the inhabited earth, understood in this context as belonging to the Universal (catholic) Church, which we as Orthodox Christians belong to. The ecumenism that we reject is one in which all Christians are considered to be on equal footing or share equal amounts of theological rectitude - a teaching we absolutely reject. The below chart is a handy guide to the Seven Ecumenical Councils and what they accomplished: