There are not a few Orthodox who are confused about the place of their Church in the 20th century and the religious milieu through which She is passing. Part of their concern (i.e., those who think seriously about their religion) is the question of what ought to be our relationship with the non-Orthodox. For such a long time, the Church was friendly towards but not ever intimate with the various Christian denominations. But now many Orthodox bishops and priests are saying and doing things which some of us do not understand and, some of us, do not wish to hear and see.
No longer are the Orthodox told that their Church is the true Church, the Catholic Church, the Church of the Prophets, Apostles and Fathers. And, indeed, we are no longer given the advice,
“Keep away from all heretics and schismatics, neither pray with them nor attend their religious meetings and services.” 
There appears to be a contradiction here. Beliefs which have been held to be true and unchangeable are now suddenly being challenged by those sworn to uphold them. New ideas seem to be entering a Church which had always boasted that She has never added to nor subtracted from the Apostolic Faith.
In part, our present confusion is the result of worldly temptations, but more unhappily it is the consequence of ignorance: the Orthodox do not know what the Church is nor what Her relationship with non-Orthodox ought to be. Many of us do not know that the Church the Lord established is One, because God is One and there is but one Messiah; Her Faith is one, because Christ did not leave humanity numerous and conflicting sets of belief; and there is but one Baptism, because there is but one Church into which a man is incorporated.
Of course, there are those who insist they are members of the Church of Christ despite the fact that, if their claim were true, the Church would no longer be trustworthy nor worthy of obedience. Why obey what cannot be trusted? Why believe what is doubtful? Why belong to a fellowship which could mislead us? To be sure, if the teachings of the Church are wrong, then, why not find “my own truth?” But Jesus Christ did render His Church incapable of error and He gave Her the Holy Spirit to that end (John 15: 26). It follows, then, that all those who disagree with the religious teachings of Orthodoxy are “heretics” and their false ideas “heresy.”
This language may “turn off” some people, but it is only because they do not know what is meant by the terms “heretic” and “heresy” and the necessity for them. The explanation above is only partial; perhaps, a few more details will help us to understand why these words have been in the theological glossary of the Orthodox Church from the beginning.
A “heretic” is simply one who maintains a “heretical doctrine.” The sincerity and good will of the “heretic” is not in question. Nevertheless, “heresy” is evil, because it is a powerful means by which the Devil seeks to “prevail” against the Church — something he will not do, according to the Lord’s promise (Matt. 16:18). It must be distinguished from “splitting”, “factions”, and “disputes” which occur within the Church (Gal. 5: 20). Heresy, at first, is a “leaving” or “falling away” from the Church (apostasia) and eventually hardens into a “sect” (hairesis). One may also be expelled from the community of believers on account of false teachings. The Scriptures call “heretics” “false teachers,” “denying the Master,” “deceivers.” 
Listen to what Saint Ambrose of Milan says about “heresy” and “heretics”:
“Or, like some dread and monstrous Scylla, divided into many shapes of unbelief, heresy displays, as a mask to her guile, the pretense of being a Christian sect, but those wretched men who she finds tossed to and fro on the waves of her unhallowed strait, amid the wreckage of their faith, she, girt with beastly monster, rends them with cruel fang on her blasphemous doctrine” (On the Christian Faith, book I, chapter 6, 46).
And St Cyril of Jerusalem says:
“For men have fallen away from the right Faith… And formerly the heretics were manifest, but now the Church is filled with heretics in disguise, for men have turned from the truth and have itching ears (II Tim. 4:3).” (Catech. Lectures XV, 9).
In other words, heretics are those who cling to false doctrines. They have always existed and always will (Matt. 17:7). Some have left the Church and formed a sect.” The “sect” becomes a historical entity which propagates itself, standing as a rival to the Church. The Church takes names— “catholic,” “orthodox,” “true,” “apostolic,” etc.—in order to distinguish Herself from them. She generally gives those “sects” the name of their leader or first principle (e.g., the Arians after Arius, the Nestorians after Nestorius, the Iconoclasts after Iconoclasm, Papists after the followers of the Pope, Calvinism after John Calvin, Lutheranism after Martin Luther, etc.) “Protestantism” is the collective name of those “sects” which emerged from Papism in the 16th century.
There are, too, heretics who act as if they were members of the Church, the Orthodox Church, while in fact they are not. They are “wolves in sheep’s clothing,” “tares among the wheat,” “heretics in disguise.” These are most common in an age of social, political and theological turmoil (e.g., during the 4th century merger of the Church with the Roman Empire, the Crusades, the Turkish domination, the Bolshevik Revolution, post-World War I and II, etc.). In our own day, we have “ecumenists” who have denied that the Orthodox Church is the Church of Christ. Although they masquerade as bishops and priests (and laymen), they are really no longer members of the Church. They can be recognized by their conduct contrary to the canons and an attitude contrary to the spirit of the Fathers. They have not departed or been expelled, because they are deceived and deceiving.
Now that we have learned, in very general terms, what “heresy” and “heretic” are, what specifically is their crime? St Cyprian compares heretics to “the sons of Aaron, who placed strange fire (heresy) upon the Altar of God,” that is, “despising God’s Tradition, seek after strange doctrines and bring in teachings of human appointment” (On the Unity of the Church, 18-19). Similarly, St Irenaeus says that heretics do not follow the Apostolic Tradition, Christ’s teachings “derived from the Apostles” and delivered, “to our time by means of the succession of bishops” (Against Her. III, 3:2). And, indeed, there are no bishops without that Tradition and no Tradition without bishops.
What is the Tradition of the Apostles? It is the religious truth revealed by God to the Prophets which foretold Jesus Christ; and when He came, fulfilling the prophetic types and beliefs, the Lord taught the fullness of truth to His Apostles. They gave that truth to the Church. The Fathers witnessed to it, the Councils formulated it, the Liturgy embodied it for worship, the bishops taught it and the Faithful defended it. The Apostolic Tradition is, then, “the faith of Jesus Christ” handed over, continuously and unchanged, from one generation of Orthodox Christians to another, the Holy Spirit guiding its transmission and protecting its contents—despite the malice of Satan and the weakness of men.
Again, departure from the Apostolic Tradition is “heresy” and, therefore, along with the ancient “sects,” the Orthodox Church counts Papists and Protestants as “heretics”—the recent declaration of some Orthodox “ecumenists” notwithstanding. One need only read the sermons and treatises of Orthodox theologians from St Photius to St Mark of Ephesus. Likewise, the rejection of Lutheranism by The Three Answers of Patriarch Jeremiah II (1567) or the condemnation of “all Western innovations” by the Councils of Jassy (1642) and Jerusalem (1672). The Orthodox Councils of the 18th and 19th century make it clear that Protestants and Papists are heretics as do the encyclicals of the Eastern Patriarchs (1848, 1896). In 1904, the Holy Russian Synod urged Western Christians to come to the Orthodox Church, “the Ark of Salvation.” Two years before, the famous Joachim II, the Ecumenical Patriarch declared, “Our desire is that all the heterodox [heretics] shall come into the bosom of the Orthodox Church of Christ which alone is able to give them salvation.” Until very recently, the Church has made the same statements at WCC meetings (e.g., Oberlin).
To summarize, then, “heresy” is an error in theological teachings, a turning away from the Apostolic Tradition, which involves a loss of membership in the Church of Christ whether by “falling away” or “expulsion.” Heretics usually form “sects” which become historically competitive to the Church. From these “sects” others may or may not emerge. “Heresy” must be distinguished from “schism” or internal disputes, factions or groups within the Church which have seceded from the lawful government of the Church. “Heresy” and “heretics” have existed from the time of the Apostles and exist today whether as “sects” outside the Church or “heretics in disguise” within the Church (or so it appears). Yet, to use the words of St Ambrose, both “apostates” and “sectarians” are veritatis inimici (inimical to the truth) and impugnatores fides (assailers of the faith). They are not just those who leave the Church, but those who are born in heresy long after the first dissenter departed with his private theological opinions.
The 4th Commandment of the Eastern Orthodox Church as stated in The Prayer Book for Eastern Orthodox Christians, compiled by the Rev. Peter H. Horton-Billard and the Rev. Vasile Hategan; translated and edited by the Very Rev. Michael C. H. Gelsinger. New York, 1944, p. 15. This collection of Orthodox prayers and devotions was distributed to the Orthodox in the Armed Forces of the United States during World War II. This commandment is a summation of admonitions from the Bible, the Holy Fathers and the Canon Law of the Orthodox Church.
[2}On “heresy” and “heretics”, see Tit. 3:10; Gal. 1:8-9; Heb. 13:9; II Peter 2:1; II John 7; Jude 4.