While there are several books that provide excellent instruction and teaching about Orthodox Christianity, the Orthodox Catechism web site of Greek Orthodox Metropolitan Archbishop SORTIRIOS is an excellent resource for study.

Another resource is provided in a glossary format of Orthodox terminology by the Antiochian Orthodox Christian Archdiocese.

What do Christians believe?  What do Christians teach?  What must we believe to be Christian?

In an effort to establish the "bottom line" of Christianity during a time when gnostics, pagans, and "wolves in sheeps clothing" were introducing or spreading false and heretical teachings about God, about God's Son Jesus Christ, the teaching of Christ, the Church, etc., it became necessary for the Fathers of the Church -- those who had remained faithful to the teaching of Christ, the Apostles, Holy Scripture, etc. -- to establish a Statement of Faith.

This "Statement of Faith" provideds the basics, that "bottom-line," of Christianity.  It was developed by the clergy and theologians of the Church at the first two Ecumenical Councils, the Council of Nicea in 325 AD, and at Constantinople in 381 AD.  This Statement of Faith is known today as the Nicene or Niceno-Constantinopolitan Creed, and is referred to by Orthodox Christians as our "Symbol of Faith."

By conciliar agreement no part of the Creed was to be altered or changed, no additions or deletions, without the agreement of all at another Council.  Unfortunately, for what may have initially been valid concerns, the Creed was changed in the Western part of the Church in the sixth century.

Later, for a variety of not-so-pious and justifiable reasons, the Western or Roman part of the Church attempted to force the unauthorized change upon the remainder of Christiainty, the Ancient Christians centers (Sees) of Constantinople, Alexandria (Egypt), Antioch, and Jerusalem, and their "daughter" Churches.

The addition, known as the "Filioque," are three seemlngly little words ("and the Son") that ultimately contributed to the separation of the Latin or Western Roman See from her sister Sees of Constantinople, Alexandria, Antioch, and Jerusalem, and introduced theological errors regarding the realtionship of the Father, Son, and Holy Spirit.

The Filioque, along with other arbitrary changes from the practice and teaching of the Early Church -- as well as the attempts of one Bishop (Pope) of Rome after another seeking authority and power over all of Christianity -- contributed to what we know today as "the Great Schism," the separation of the Western Church (the Roman Catholic Church) from the Eastern Church (the Greek* or Eastern Orthodox Church).

For more about the the disasterous change in the Creed by the West, read and study the History of the Filioque.

* "Greek" as regards to the language of the Church, not ethnicity.  The New Testament books were written in Greek.  The Early Fathers of the Church wrote in Greek.  Greek was the "international language" of the world during the first millennium of Church history, just as English is considered the international language of the world today.  Staying faithful to the Early Christian Church, Greek remains today the "official" language of Orthodox Christianity, although the language used within most Orthodox Churches is that of the nation or country in which the local churches exist.