On the one hand, we can find lineaments of ecumenicity in the apostolic Council of Jerusalem in Acts 15. On the other hand, according to John Meyendorff, "ecumenical councils" are the direct historical product of the distinct relationship between Church and Roman Emperor:
"throughout the fourth and fifth centuries, the bishops could not agree among themselves. Council succeeded council, issuing formula after formula, so that, in order to assure unity, the emperors were, in fact, forced into choosing between ecclesiastical factions and, therefore, interpreting conciliar formulas themselves. There are many occasions showing that Constantine and his successors stepped in with great reluctance into the role of interpreters of Christian theology and church discipline, and that Christians called on them to assure that role. The most famous case in point is Constantine's role in settling the Donatist dispute in Africa. His personal decision in favor of the Catholic party followed lengthy and persistent attempts at having councils--in Rome, then Arles--to resolve the dispute." Imperial Unity and Christian Divisions, p. 34.