You seem to have completely missed the point.
That being that Catholic was the first denominational named group of Christians.
That simply isn't true There were many references to specific Christian groups named in the Bible. Have a look at Revelation 1-3, where the groups were addressed by physical location, just as the Catholic denomination is referred to as the church at Rome.
The history of the Eastern Orthodox Church is the formation, events, and transformation of the Eastern Orthodox Church through time.
According to the Eastern Orthodox tradition, the history of the Eastern Orthodox Church is traced back to Jesus Christ and the Apostles.
Here are some facts...
Until 1054 AD Eastern Orthodoxy and Roman Catholicism were branches of the same body, so they can just as well make the claim that they were "the first church".
All Christian denominations are rooted in the life and ministry of Jesus Christ and
share the same origins.
Early believers were part of one body, one church. However, during the ten centuries following the resurrection, the church experienced many disagreements and fractions. Eastern Orthodoxy and Roman Catholicism were the results of these early schisms.
Cultural differences played a major role too, with the Eastern mindset more inclined toward philosophy, mysticism, and ideology, and the Western outlook guided more by a practical and legal mentality.
In 1054 AD a formal split occurred when Pope Leo IX (leader of the Roman branch) excommunicated the Patriarch of Constantinople, Michael Cerularius (leader of the Eastern branch), who in turn condemned the pope in mutual excommunication.
In other words,
Catholic boasting is nothing but propaganda.