Is the Orthodox Church (i.e., Greek Orthodox, Eastern Orthodox) a true Christian church? Are their beliefs consistent with the Bible?
The Orthodox Church was established in AD 1045 following a schism of the Roman Empire which split the church in the east from the church in the west. The church in the east, the Eastern Orthodox church, renounced all ties to the church in the west, the Roman Catholic church, including denying the authority of the pope.
Though the Orthodox Church does not recognize the authority of the Roman pope and rejects various other Catholic practices (notably, celibacy for priests), nevertheless the church holds many of the same doctrines and beliefs as Catholicism, including a false gospel preaching mysticism and human works as the means to salvation in keeping with the teaching of the Catholic Church. In fact, the official name of the Eastern Orthodox church is the Orthodox Catholic Church (as opposed to the Roman Catholic Church).
Of most concern, the Orthodox church holds that salvation is not by faith alone in Christ, but like Catholics, they teach that other requirements (i.e., penance, good works) are required for salvation. Essentially, the Orthodox church's teaching on salvation is the same as Catholic teaching in this area.
Secondly, the Orthodox church venerates Mary as do Catholics, calling her the "Most Holy Mother of God." Mary is praised in their hymns as "more honorable than the Cherubim and beyond compare more glorious than the Seraphim," suggesting that Mary (a human being) is higher than angels, which directly contradicts scripture (see Psalm 8:4-5).
Going even further, the church says Mary is the "Mother of the Church — it is in Her person that the Church glorifies motherhood.” Notice they even capitalize any pronoun referring to Mary as if Mary were equal with deity. To glorify Mary in this way is nothing short of blasphemy, as God does not share His glory with anyone (see Isaiah 42:8). Again, this is very Catholic.
The Orthodox church equates church tradition with scripture, including erroneously holding that water baptism washes away our sins, at least initially. To quote official church teaching:
Finally, the Orthodox church venerates (i.e., assigns spiritual power to) the dead, calling them "saints" and praying to them for spiritual blessing. As with Catholics, this practice is blasphemous and contrary to Scripture.
There are other similarities with Catholic doctrinal, but these serious departures from Scripture are certainly sufficient for us to conclude that the Orthodox church is a false church teaching a false Gospel that cannot save.
We must allow for the possibility that from time-to-time true Christians may be found attending the Orthodox Church. Like the Catholic Church the Orthodox Church does not preach the true Gospel nor does it practice biblical Christianity, but that does not mean God's Spirit is incapable of bringing true faith to someone within this community. Nevertheless, should a member of the Orthodox Church come to saving faith in Jesus Christ, we would say the person was born again despite the church's negative influence, not because of the church.
Furthermore, we encourage all born-again Christians to seek fellowship in churches that hold to the true Gospel. Remaining closely tied to a false church can have serious, negative influences on the believer's walk of faith.