A.: You and I could concur on papal infallibility, but we would not thereby achieve much of a union. The great challenge is not to pick and choose individual beliefs, but to bridge the gap between our respective Churches as religious communities with long histories and deep memories. The estrangement between Orthodoxy and Catholicism is over a millennium old. It has been caused not only by doctrinal differences, such as opinions over the role of the Pope, but also by historical events, especially the Pope-sanctioned Crusades; during these, Orthodox lands were ravaged and Latin episcopates set up in the East to convert Orthodox Christians. Such wounds take time to heal. This healing process began only about a generation ago, with an ecumenical meeting between the Pope and the Patriarch of Constantinople in Jerusalem. It will take many more generations of patient dialogue and healing between the communities before substantial results could emerge, by God's grace.
The first need is to reject the way of polemics and instead embrace a prayerful path to fulfill Christ's prayer "that they may all be one" (Jn 17:21). Christians were commanded in the Sermon on the Mount to love even their enemies (5:44). It is to our shame that we have fostered contempt, prejudices, conflicts and even war against each other. The current Pope's recent gesture of asking for forgiveness for historical wrongs committed by Roman Catholics, even though he did not mention the Crusades, is an important step toward authentic dialogue and healing.
The other important need is for theologians and church leaders to continue to work on resolving the divisive doctrinal issues while acknowledging that we share a common, rich tradition of Christian antiquity. The main doctrinal difference that divides us is indeed the role of the Pope. It is the one which actually caused the official break between the two Churches in 1054. From the Orthodox viewpoint, the Pope may be accorded a "primacy of honor," meaning the right to call and preside over meetings and councils, as well as the right to be the spokesman for the Church. But he is not given a primacy of legal power--or accorded infallibility.
A second crucial difference is over the interpretation of the Nicene Creed, specifically a clause that was inserted by the Western church, often referred to as the "filioque clause," meaning "and from the Son." When reciting the Creed, Orthodox Christians say that the Holy Spirit proceeds from the Father, while Catholics say the Holy Spirit proceeds from the Father and the Son. This "and from the Son" phrase was improperly inserted, Orthodox believe, into the Creed's article on the Holy Spirit. Not only did the Eastern Church believe it was a break with the traditional interpretation of the Holy Trinity, that is, the relationship of the Holy Spirit to the Father and the Son, it was also an assertion of authority on the part of the Latin church, and eventually by the pope himself, against the universal authority of the Second Ecumenical Council (381 AD)--an illegal act rejected by the Eastern Church.
If these two key doctrinal differences could be resolved, a solid theological foundation, as well as a deposit of trust, would be established to deal with other theological disputes (for example, purgatory, merits, indulgences, and the understanding of priestly authority). Only then can we continue on the long process of reconciliation and hope-for unity.