A Liturgy is a construction of doctrine and ritual used to direct organized/orderly worship by a particular Church (in some ways even modern Evangelical Mega-churches have a liturgical structure). You will see at the following link that each of these written Orthodox, Catholic (and Coptic, Assyrian Church of the East / Nestorian , Liturgy of the Holy Apostles (Armenian),etc), Qurbana (Indian) liturgies are uniquely beautiful but yet they each also contain the same basic elements such as; Scripture readings, prayers, songs of worship and most notably all include a common and clear belief in the sacrificial nature of the Eucharist and their common emphasis on the transubstantiation of the elements of bread and wine as they become the very real, physical presence of the Flesh and Blood of Christ.
What makes this so amazing and profoundly important to understanding the history of Christian doctrines is that the Assyrians broke from the Great Catholic Church around 430 AD, the non-Chalcedon / Monophysite Churches left around 450 AD and the Eastern Schism of the Orthodox took place about 1000 AD. Hence, we can hypothesize that these doctrines and rituals entered the Church very early and not through a Roman Catholic Apostasy of the Middle Ages as Protestants often wishfully state. It may even be suggested from this evidence that these practices even came from the Apostles and Christ Himself as the historic Apostolic Churches have been claiming from the beginning via Sacred Tradition.
2 Thessalonians 2:15 ESV
So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter.
