From where I sit, it looks pretty clearcut: it happened at Chalcedon. Let me explain why I say that.
We believe in one holy catholic and apostolic church.The ancient churches around the world founded by the apostles remained united for centuries afterwards, to the extent that we boldly confess in our creed that we believe in a church that is one (united), holy (of God and pursuing the things of God), catholic (worldwide and all-inclusive), and apostolic (with the same message of Christ brought by the apostles).
There were many ancient churches founded by the apostles, and these churches were in fellowship with each other in the early days of the church. Five of these churches had special prominence and gained reputations as centers of Christianity in their regions. Here is a brief introduction to the five major Christian centers of the ancient and undivided church.
Rome - The world's leading city in that day. Peter and Paul were instrumental in its eventual conversion to Christianity, and Rome was the place of their deaths. Home of some important early Christian writers. While all Christian churches produced martyrs, Rome may have had more than its share of confessors of the faith as it housed the throne of the pagan emperors.
Alexandria - Founded by Mark, the author of the second gospel and a disciple of Peter. Birthplace of monasticism, producer of the renowned writings of the Desert Fathers. Center of Christian scholarship in the early church. Home to an impressive number of the great Christian writers and leaders of the first four centuries.
Antioch - Founded by Peter and well-known to Paul; this may have been Luke's home town. The place where followers of Jesus were first called by the name Christians; base for an impressive amount of missionary work.
Constantinople - Early Christian origins. Distinguished itself during the Christological and Trinitarian controversies of the early church. The Nicene Creed in its present form was finalized in Constantinople (excepting the later Roman addition of the phrase "and the son" to the article on the Holy Spirit, an addition which is not universally accepted).
Jerusalem - All the apostles and also some family relations of Jesus founded this church, mother of all other churches. Home of the first church council, place of the crucifixion, the resurrection, and the great outpouring of the Holy Spirit at Pentecost. The birthplace of the church. Flourished for centuries (when not under siege) as a base for both Jewish and Gentile Christians. Held a leadership position in the earliest church.
There were other centers of Christianity which were accorded a great amount of respect, such as Ephesus and Nicea. Some of these smaller centers also had apostolic origins. The point in the sketch of the five great ancient centers is not to limit apostolic Christianity to these five sees, but to recognize the status they held in the early church.
If you start with this or a comparable thumbnail sketch of the ancient church, you can then find an objectively meaningful answer to the question of when the church's operation changed from the one, holy, catholic, and apostolic church: it happened at Chalcedon. The Council of Chalcedon in the year 451 A.D. saw the division of the apostolic churches. Alexandria and Antioch, though apostolic, were no longer recognized as part of the church by the other sees due to their disagreements with the decrees of Chalcedon over the finer points of how the divine and human natures coexist in Christ. According to the western churches, the church was no longer perceived in the same way as "one, holy, catholic, and apostolic", but now was defined in practice as "those who agree with the decisions of the Council of Chalcedon." It was quietly assumed that no one who was truly apostolic could have legitimately disagreed with these findings on the subtle interrelation of human and divine nature -- no more than they could have legitimatly disagreed with the earlier councils affirming that Christ was crucified under Pontius Pilate. I think that was the fundamental mistake that led inevitably to the now-common practice where we recognize Christian unity and limit fellowship by fine points of doctrine which may not be clearly taught anywhere in the teachings of Christ or the apostles.
So the year 451 A.D. was when the western church changed its own perception of the church. The decisions of Chalcedon were seen as the defining and unarguable arbiter of membership in the church. The phrase "one holy catholic and apostolic church" could no longer be used of any single church body beginning in that year: the apostolic churches were no longer in fellowship.
There have been no truly ecumenical decisions since then. There have been no truly ecumenical councils since then. All the doctrinal developments since then have been regional or sectarian. That is not to say that nothing worthwhile has happened; a great many accomplishments have been made all around. But the most necessary accomplishment has not happened: the reunification of the apostolic churches around the world. Earlier I said that, reviewing church history, there is an objectively meaningful answer to the question of when the church's operation changed from the one, holy, catholic, and apostolic church. I was careful not to say that the church's identity changed, but its operation. We are all the church by identity, whether or not we recognize that in our thoughts, words, and actions. The church is still holy, catholic, and apostolic -- and we are still one in Christ whether we recognize each other or not.
Until the apostolic churches speak with one voice, there is no single voice who can speak for the one holy catholic and apostolic church. But to unravel the mess we are in, we have to go back beyond Chalcedon, back to the source. We have to recognize one catholic Christianity wherever we find what is holy and apostolic.