What does it mean...

to be a Christian?

According to orthodoxanswers.org:

The Church teaches that to be Christian in the full sense of the term, one must be received into the Orthodox Church. This is why new Orthodox Christians, regardless of previous affiliation, are spoken of as “newly illumined.” According to strict, technically correct terminology, the only Christians are Orthodox Christians. Having said this, it is perfectly permissible to refer to some heterodox people who confess the name of Christ as Christian by means of economy. This means that we are still using the term, but using it in a looser sense. St. Innocent of Alaska in his famous “The Way into the Kingdom of Heaven” spoke of “non-Orthodox Christians.”

We must emphasize, however, that Christian does not necessarily mean saved. There are many Orthodox Christians, rightly baptized and received into the Church, who will find themselves in Hell when the verdict is pronounced at the Dread Judgment Seat of Christ. Likewise, there are likely many non-Orthodox who, though they are not presently united to Christ’s One Church, will be united to her by God’s mercy in the life to come.

While not judging the destiny of any individual (apart from those venerated in the Church as Saints), the Church calls all men to cleave to Christ through her. Salvation is difficult enough when one’s life is spent in the Church. It is undoubtedly more difficult when one’s life is spent outside the Church.
To sum up, Christians from other confessions can be called “Christians” as they profess the name of Jesus, however, Christian in it’s fullness means to a member of the one Holy Catholic and Apostolic Church, namely the Orthodox Church.

However, just because someone is Orthodox Christian, doesn’t mean that he’s automatically saved. An Orthodox that is baptized may find himself in hell, and a non-Orthodox or even a non-Christian that had never the opportunity to learn about the Orthodox Church and become a member, may be united with God. That’s why we can’t say who will be saved and who not, this is totally up to God.

It seems very odd how different but similar beliefs can divide so many so far away from each other...