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There is another eBible question relevant to this one:
Why would you need to "witness" to Eastern Orthodox Christians who already follow Jesus Christ?
@Michael Sisson it's because many (or even most) Eastern Orthodox people rely on religion for salvation, like believing they are saved because they are a member of a church, that they pray to saints (Like Mary), or that they have done sacraments; rather than basing their salvation on the work of Christ. In summary, they rely on good works for salvation rather than Christ alone, similar to the teachings of the Roman Catholic Church.