As I mentioned last time, when the split finally came between the Eastern and Western Churches in 1054, the Filioque was the theological rationale for it in Constantinople.
Again, it was very much like a divorce where differences that had once been tolerable became life and death quarrels. After centuries of perfectly acceptable teaching about the procession of the Spirit from the Father and Son to which very few had objected, Eastern theologians began to give an extremely stringent reading to Councils that had been held 600 years before at Ephesus and Chalcedon.
These councils had forbidden the introduction of another Creed, and had imposed penalties on any who broke this law. Their point, of course, had been to prevent partisans in the arguments of their day from proposing some “counter-creed” to overturn the results of the Council, not to forbid henceforth all theological development forever. So they had not forbidden anybody to explain the same faith or to propose the same Creed in a clearer way. How could they since that is exactly what their own councils had done? So for centuries afterward, in East and West, the Church could (and did) teach explicitly what was only implicit in apostolic teaching or clarify some point of teaching in response to questions that had not arisen earlier in the life the Church. That is exactly why, for seven centuries, people had talked freely about the procession of the Spirit from the Father and the Son. But, suddenly, in the 11th century, these ancient conciliar prohibitions were given a new meaning under the lash of increasing acrimony between East and West and the idea of the procession of the Spirit from the Son was deemed heretical in the East.
Nonetheless, the real issue was not that the Filioque was either new or contrary to Tradition. Rather, the real sticking point for the East was the perceived arrogance of Rome in unilaterally changing the words of the Creed.
As the centuries rolled on, the Western Church essentially took the position that, however the Filioque was arrived at, there was certainly nothing false to apostolic teaching in it and that it did indeed express the heart of that teaching: God is love. At the Fourth Lateran Council in 1215, the Filioque was defined as a dogma of the Church and formally added to the Creed by all the Churches in union with Rome at that time. The Eastern Churches, meanwhile, for a host of reasons pertaining far more to historical wrongs suffered by them (including the sack of Constantinople by Catholic Crusaders, leading to the eventual fall of Constantinople to Islam) continued to see in this development an expression of Roman arrogance. Both were right. As is common in such divorce proceedings, both sides had legitimate points, and both sides were at fault.
All that said, this book is not intended to be a full history of the Church nor to settle a dispute between two great civilizations that has lasted a thousand years. I aim to focus only on the Creed and its implications for us today. As a convert to the Catholic faith, it is obvious which side of the controversy I think has the better theological claim, but I do not think that obliges anybody to condemn the Eastern Churches. Indeed, thanks be to God, the rift between the two great theological traditions of East and West has finally started to heal a bit (though there is a long way to go on both sides). Pope Paul VI and the Patriarch Athenagoras I of Constantinople lifted the millennium-long mutual excommunications of Catholic and Orthodox Christians in 1965. And conversations between the Catholic Church and other theological traditions, including the Orthodox, are ongoing.
In addition to this, Rome has sharply revised its former approach to Eastern Catholics in its own communion. As we shall discuss in more detail in Chapter 13, there are within the Catholic Church different “rites”—that is, ways of celebrating the liturgy and sacraments—which grew up and evolved in various cultures and places. The most common rite is called the “Latin rite” and is what has been celebrated in western Europe for centuries. Meanwhile, in the centuries since the Great Schism, various Christians in the Eastern Orthodox Churches were reunited with the Catholic communion in union with the Pope. These former Orthodox Churches had their own sacramental, liturgical, and cultural traditions and Latin tradition was foreign to them. But, among other things, Rome required them to recite the Filioque, which they had never done before, as a sign of their acceptance of the dogma. Many Eastern Catholics, even though they accepted the dogma, felt this to be overbearing and it was a source of friction within the Catholic Church.
In recent years, however, the pope and bishops of the Western Church have regretted this ham-fistedness and asked the members of those rites to “return to their roots” by observing the custom of the Eastern tradition whence they came and using the wording of the Creed as it stood after the First Council of Constantinople in 381—without the Filioque. Relatedly, and especially since the Second Vatican Council, it has also become the habit of the Catholic Church to respect Eastern Orthodox sensibilities regarding the Filioque. So, for instance, in a document on the Church’s relationship with other religions called Dominus Iesus, Pope Benedict XVI quoted the Creed without the Filioque, out of respect for Eastern Christians. Similarly, in ecumenical gatherings with representatives of Eastern Churches, the Filioque is omitted when the Creed is recited.
How can the Church do this? Is it not repudiating its own dogmatic teaching from the Fourth Lateran Council? Not at all. Just as you can recite the Apostles’ Creed without rejecting the Nicene-Constantinopolitan Creed, so returning to the language of the Creed as it stood from 381 till 1215 is not a denial of the dogma of the Filioque. It is simply an act of respect for those who honor an earlier form of the Creed.
 You can read the document celebrating that at https://w2.vatican.va/content/paul-vi/en/speeches/1965/documents/hf_p-vi_spe_19651207_common-declaration.html.
 Joseph Cardinal Ratzinger, Dominus Iesus 1 (2000). Available on-line at http://www.vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_20000806_dominus-iesus_en.html as of September 19, 2019.