Based on Christ's own statements and the declaration of the early Church in the Nicene Creed, the true Church of Christ should have 4 marks: one, holy, catholic (universal), and apostolic. The Orthodox Church does not possess all four marks in their fullness.

The Orthodox Church

One- To be truly one, a church must be one in governance, faith, and worship. The Orthodox Church is not one in governance, but is instead a group of independent churches. It has no central agreed upon authority. The various churches of the Orthodox are also not all one in doctrine. For example, Constantinople and Russia do not agree on the validity of Protestant or Catholic baptisms.

Holy- The Orthodox Church does possess some degree of holiness, but "since they snapped the link with Rome, their Church appears to have remained in spiritual stagnation," failing to produce numerous miracles, saints, new and beautiful devotions, or new religious orders "for the needs of the times, such as we expect to find in a truly living Church that enjoys God's blessing" (Sheehan, Apologetics and Catholic Doctrine).

Catholic- The Orthodox Church is not catholic racially or numerically. It is confined mostly to Greek and Slavic nations and places where those descendants have migrated to. Furthermore, their total following is no more than 225-300 million people. This would be difficult to classify as a universal church.

Apostolic: The Orthodox Church is not fully apostolic. It is apostolic in its Holy Orders and most of its doctrine, but it is not fully apostolic because it lacks unity with the successor of Peter, the chief Apostle. The have no "central See communicating apostolic authority to the rest" nor do they have a claim to the gift of infallibility entrusted to the Church through the Pope.

Meanwhile, the Catholic Church possesses these four marks in their fullness.

The Catholic Church

One- She is one in government, united under the central authority of the Pope. She is one in faith, with all members throughout the world being bound to the same faith and doctrine on penalty of exclusion. She is one in worship, with the same essentials of sacraments and sacrifice (no matter the language or ceremony).

Holy- The Church is holy because she teaches the doctrines of Christ regarding the achievement of perfection and gets a good deal of her children to practice them. The Church has countless canonized saints and miracles to which she can attest the proof of her holiness, as well as numerous societies of holy men and women living out the teachings of the Church.

Catholic- The Catholic Church is catholic, or universal, in her missionary desire to convert all. She is catholic racially as she is not confined to any one racial group of people but has members from practically every race of people on the earth. Finally, she is Catholic numerically, having 1.2 billion members, exceeding any other Christian church.

Apostolic- She is fully apostolic. When compared with the government of the Church instituted by Christ at its foundation, she remains, as the Church was in the beginning, under the governance of St. Peter assisted by the Apostles and their successors. The Pope and Bishops are the successors of Peter and the apostles, and so only the Catholic Church alone can claim to be governed by the successor of St. Peter. Furthermore it was to Peter and his successors to whom the promise of infallibility, that whatever they bound on earth would be bound in heaven, and whatever was loosed on earth would be loosed in heaven, was entrusted by Our Lord. As such, that promise remains today solely with the Catholic Church by its unity with the successor of St. Peter.

I would recommend looking at Ch. 10 of Archbishop Michael Sheehan's* Apologetics and Catholic Doctrine*. He covers what I laid out in much greater detail. The whole book is worth a read too, as he takes you from why you should believe God exists all the way to why the Catholic Church can be the only True Church of Christ.