While previous books I've read (Roman but not Catholic; Upon this Rock) reflect the ecclesiological debate between Protestants and Catholics, this volume primarily focuses on the debate between Roman Catholics and Orthodox Christians. This is important because it determines the subject matter.
The sources of Protestant doctrinal revelation start and end with the Holy Scriptures. Yet for Orthodox and Catholics, these sources begin fundamentally with the authoritative Church Christ established in the apostles. This Church then recognises the scriptures with apostolic authority, and serves the church through the passage of time with the authority of Christ (again, apostolic authority), defending and expounding the orthodox interpretation of the Holy Scriptures against heretical interpretations.
Both Catholics and Orthodox believe that the Apostles had successors who carried on their authority. The question is, how is this authority exercised? What does it look like when Christ is definitively instrumentally speaking through the Church? Thus, a deep analysis of history is necessary; particularly the first millennium of the Church.
Modern Orthodox believe that the Church can only speak as one. So ecumenical councils are the only way to come to decrees which represent this. The catholic Church, even since pre-Nicea, always insisted that councils do not carry definitive authority from Christ unless the Bishop of Rome approves of them.
Why? Because Christ directly bestowed upon Saint Peter a jurisdiction over the entire Church in the *Tu es Petrus* (What you bind will be bound in Heaven...) bestowal of authority. And because Peter was Bishop in Rome, his successor inherited his authority. Thus, the Bishop of Rome sits in the "seat of Peter" and has the authority to make theological rulings which represent the voice of Christ.
What is virtually undeniable by any honest look at history (as Ybarra shows very thoroughly) is that the Church has been deeply consistent (since the beginning) in understanding Rome to be the ecumenical See which was "judged by no one". This was not rationalised because of the fact that it was the capital of the Roman Empire, because 1) the reason provided for it always had to do with the a prior bestowal of jurisdictional authority to Peter from Christ, and 2) Rome continued to be recognised for its divine commission and ultimate theological jurisdiction even after the West fell and the Capital was moved to the East in Constantinople.
There is however a problem for Orthodox Christians, who deny any a-prior authority to Peter and place it rather on a Church-wide level. This recognition is also deeply rooted in Eastern Saints proclamations as well. Eastern saints and patriarchal leaders have a consistent history of appealing to the Pope in Rome to resolve their issues -- as the voice of the Apostle Peter who is alone given the authority of Heaven in resolving theological disputes. These distinct recognitions of papal authority also made their way into the acts of ecumenical councils, which Orthodox hold to be definitive rulings of God.
Orthodox have a history of interpreting these statements as not genuine, but merely flattering Rome for the sake of achieving a "modus vivendi" and political peace. But considering that those Saints which testified and appealed to the authority of Peter throughout history, even to other Eastern saints, this seems like a very shaky theory.
Ybarra covers a number of other reasons why the theological system of Eastern Orthodoxy rests upon shaky theoretical foundations. For example, they previously relied on the Emperor to call councils and the Pope to confirm them, yet now have neither of these. Alas, there is no more emperor, and they have "genetically removed" the papal doctrines with which their history is so rich. They are therefore disabled when it comes to coming to any rulings to resolve the doctrinal conflicts between the various eastern churches.
I won't go into everything. One last thing I want to mention is that I do feel that Ybarra generously covered a number of Orthodox arguments, and I feel that I understand the Orthodox side in much more depth than I did before. Of course, this is no replacement for reading directly from Orthodox theologians.
Ybarra is quick to concede any demonstrable weakness with the Catholic position regarding history - and there are a few weak points (Vigilius, Honorius). But he argues that when one places all the evidence on the scale, the Catholic case is still more powerful. Many Protestants and Orthodox have concluded the same over time and decided to make the impossible decision to convert to Catholicism.