The Orthodox Church is the only Holy, Catholic (concerning all of Christendom), and Apostolic Church as it was handed down to us from Christ, to the apostles and from the apostles to us. The Apostle Paul tells us that the doctrines taught by Christ and His disciples are to be safeguarded by "the Church, the pillar and ground of that truth." (1 Ti 3:15). In the first 1,000 years all of Christianity was Orthodox. The Orthodox Church went through seven ecumenical councils that firmly established the foundation of the one true Christian Church.
This link will bring you to a page on the Antiochian Orthodox Christian Archdiocese website which provides a bit more detail along with some very helpful information and sources.