Orthodox Christianity refers to the set of doctrines which were believed by the early Christians. A series of Ecumenical Councils, also known as the First seven Ecumenical Councils, were held over a period of several centuries to try to formalize these doctrines.The most significant of these early decisions was that between the Homoousian doctrine of Athanasius and Eustathius (which became Trinitarianism) and the Heteroousian doctrine of Arius and Eusebius (called Arianism).