Here are some quotes from Joseph Ratzinger's 1982 book, Principles of Catholic Theology, pp. 196-9:
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The West may point to the absence of the office of Peter in the East--it must, nevertheless, admit that, in the Eastern Church, the form and content of the Church of the Fathers is present in unbroken continuity. The East may criticize the existence and function of the office of Peter in the West, but it must also be aware that, because of it, no other Church exists in Rome than that of the first millennium--of the time when a common Eucharist was celebrated and when but one Church existed.
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Against this background we can now weigh the possibilities that are open to Christian ecumenism. The maximum demands on which the search for unity must certainly founder are immediately clear. On the part of the West, the maximum demand would be that the East recognize the primacy of the bishop of Rome in the full scope of the definition of 1870 and in so doing submit in practice, to a primacy such as has been accepted by the Uniate churches. On the part of the East, the maximum demand would be that the West declare the 1870 doctrine of primacy erroneous and in so doing submit, in practice, to a primacy such as been (sic) accepted with the removal of the Filioque from the creed and including the Marian dogmas of the nineteenth and twentieth centuries. As regards Protestantism…
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How, then, are the maximum demands to be decided in advance? Certainly, no one who claims allegiance to Catholic theology can simply declare the doctrine of primacy null and void, especially not if he seeks to understand the objections and evaluates with an open mind the relative weight of what can be determined historically. Nor is it possible, on the other hand, for him to regard as the only possible form and, consequently, as binding on all Christians the form this primacy has taken in the nineteenth and twentieth centuries. The symbolic gestures of Pope Paul VI and, in particular, his kneeling before the representative of the Ecumenical Patriarch were an attempt to express precisely this and, by such signs, to point the way out of the historical impasse. Although it is not given us to halt the flight of history, to change the course of centuries, we may say, nevertheless, that what was possible for a thousand years is not impossible for Christians today. After all, Cardinal Humbert of Silva Candida, in the same bull in which he excommunicated the Patriarch Michael Cerularius and thus inaugurated the schism between East and West, designated the Emperor and people of Constantinople as “very Christian and orthodox”, although their concept of the Roman primacy was certainly far less different from that of Cerularius than from that, let us say, of the First Vatican Council. In other words, Rome must not require more from the East with respect to the doctrine of primacy than had been formulated and was lived in the first millennium. When the Patriarch Athenagoras, on July 25, 1967, on the occasion of the Pope’s visit to Phanar, designated him as the successor of St. Peter, as the most esteemed among us, as one who presides in charity, this great Church leader was expressing the essential content of the doctrine of primacy as it was known in the first millennium. Rome need not ask for more. Reunion could take place in this context if, on the one hand, the East would cease to oppose as heretical the developments that took place in the West in the second millennium and would accept the Catholic Church as legitimate and orthodox in the form she had acquired in the course of that development, while, on the other hand, the West would recognize the Church of the East as orthodox and legitimate in the form she has always had.
Such a mutual act of acceptance and recognition, in the Catholicity that is common to and still possessed by each side, is assuredly no light matter. It is an act of self-conquest, of self-renunciation and, certainly, also of self-discovery. It is an act that cannot be brought about by diplomacy but must be a spiritual undertaking of the whole Church in both East and West...
