How is one to determine the â€śTrueâ€ť Church? Greek Orthodox? Catholic Church? Both are comprehensive but the Catholic Church seems to be more so.
The Greek Orthodox Church is one of many Orthodox Churches such as the Russian, the Serbian, the Romanian, and the Bulgarian, all of which make up one Orthodox Church united in faith and sacrament, known also as the Eastern Orthodox Church, and whose full name according to the Creed is: One, Holy, Catholic, and Apostolic Church. The Orthodox Church and the Roman Catholic Church both claim the title of â€śTrue Churchâ€ť and both share substantial continuity with the ancient Apostolic Church in terms of the sacraments, the Nicene Creed, the Saints, and the tradition of monasticism, although even in these areas they have serious differences of interpretation.
The Protestants have no single Church but literally hundreds of denominations which have broken with the ancient Christian tradition in serious ways as in the case, for example, of the sacraments, the saints, and monasticism. Therefore, apart from some small sectarian Protestant groups, each of which may paradoxically claim the title, no main Protestant denomination claims â€śTrue Churchâ€ť for itself. Instead the Protestant denominations generally hold to the â€śBranch theoryâ€ť which advocates that the â€śTrue Churchâ€ť is spiritual and invisible, and that the many denominations are historical branches or expressions of the invisible Church.
Most Christians determine the answer to your question by means of simple faith: they affirm as useful and authentic the Church to which they are born into or somehow have been connected with and find valuable whether by chance or through friends or marriage. An earnest and persistent seeker, who is willing to do extensive study and serious thinking about your question, can labor for the answer through careful examination of the following: 1) the history of the Ancient Church and its continuity with the Bible; 2) the classic theology of the Ancient Church and its connection with the Bible; 3) the faith and practice of a contemporary Church as compared to the faith and practice of the Ancient Church; and 4) the effectiveness of the worship and life of a given Church today.
Generally speaking, the Orthodox Church possesses the greatest degree of identity and continuity with the faith, sacraments, and practice of the Ancient Church. The challenging question for Orthodox Christians is not only what treasures of prayer and teaching they have inherited by tradition but also what they do with those treasures today. The whole matter is not only possession and bragging rights but also performance and vibrant witness in advancing the saving work of Christ today.