The Council of the Holy Fathers
(various Fathers with St. Constantine the Great, holding the Nicean Creed)
Question: "How does doctrinal authority work in Orthodoxy? In a simplified form, how do I know what Orthodox believe? Less simply, what are the common sources for Orthodox when seeking to believe what the Church teaches? And how is it possible to know that certain teachings are definitely the Orthodox position, not only a possible opinion?"
There are different sources of doctrinal authority in the Orthodox Church: 1. Scripture; 2. Apostolic Tradition; 3. Ecclesiastical Tradition; and 4. the living witness of the Church.
We believe that the Scriptures are the inspired and inerrant word of God, and Scripture is the core of the Orthodox Tradition. However, while we can distinguish Scripture from the rest of the Tradition, we cannot properly understand Scripture outside of the context of that Tradition.
Apostolic Tradition has its origins in Christ Himself, and is preserved in a number of different ways. For example, many aspects of Apostolic Tradition are preserved in the Ecumenical Canons. The basic elements of our worship are based on Apostolic Tradition. It is also preserved in the collective memory of the Church, and is reflected in the writings of the Fathers of the Church.
Not every canon of the Church is based directly on Scripture or Apostolic Tradition. There are also Traditions that are Ecclesiastical Traditions. The Scriptures tell us that Christ gave the Apostles the power to bind and to loose, and Apostolic Tradition tells us that this authority was passed on from the Apostles to their successors, the Bishops. When confronted with heresies or problems that are not addressed directly by Scripture or Apostolic Tradition, the Church has made decisions that are binding. The most authoritative examples of this would be the Ecumenical Canons of the Ecumenical Councils, and those local and patristic canons that those councils approved. Like the Scriptures, the Church believes that these Ecumenical Canons as well as the doctrinal statements made by these councils have an authority like Scripture, and are infallible.
While the above referenced sources of authority have greater weight, because their authority has been firmly established and universally recognized in the Church, the Church continues to have the power to bind and to loose, and so the Church makes decisions all of the time hat have authority for Orthodox Christians. For example, we cannot find in Scripture or the Ecumenical Canons a clear answer to the question of what we should make of artificial insemination, local Orthodox Churches have made statements on this question. For example, in an All-Russian Council in 2000, the Russian Church issued a document called "The Basis of the Social Concept of the Russian Orthodox Church," which addressed this question, as well as many other contemporary issues. Technically, this council would only have immediate authority over those in the Russian Orthodox Church, however, other local Churches received it favorably at the time it was issued. At some point in the future, this document may be universally received, and then have a greater level of authority than it does today, but already, Orthodox Christians outside of the Russian Church have looked to it for guidance on these issues.
It takes time for the body of the Church as a whole to come to firm conclusions about the authority of a council, or the writings of a saint. No council was had universal authority simply by virtue of it meeting with a certain number of bishops. It was only when the Church as a whole was able to reflect on such councils that they were either embraced, or rejected.
There are theological or practical matters that there is not a firm, universal answer for, and so within certain bounds, there is room for theological opinions (theologoumena) which may or may not be correct. This does not, however, mean that a person can believe whatever he wants. For example, one could have different opinions on how literally we should interpret the seven days of creation in the first chapter of Genesis, but it would be beyond the bound of acceptable opinion to suggest that the universe came into being by chance, and God did not create it.
So how does one go about acquainting themselves to what the Church teaches? You have to be Orthodox, you have to live the sacramental life of the Church, and you need to study -- study the Scriptures, the writings of the saints, the lives of the saints, and you would also do well to read good books by more contemporary authors that are recognized as good and useful texts. The longer you are Orthodox, and are actively engaged in trying to learn your faith, the more you will acquire an Orthodox mindset, and will become increasingly discerning.