Dr. Hahn's thesis can be easily dismissed by the Orthodox by pointing to the many divisions throughout Western Christianity from the split between Catholics and Protestants to the plethora of denominations dominated by particular racial and national groups. Among Protestants, there is a Church of England and a Church of Ireland. In the United States alone there are Chinese Christian Churches, Nigerian Christian Churches, Korean Christian Churches and many others. There are also specific denominations for Blacks, such as the African Methodist Episcopal churches, and churches that split into two branches over race. Both Protestant and Catholic churches in the US often have practically separate Latino congregations.
On the other hand, it is true that Orthodox churches tend to cooperate more closely with the state than Western churches do. However, this is not properly called "Christian nationalism." Rather it is the manifestation of an ancient Orthodox doctrine of political theology known as Symphonia.
The Orthodox doctrine of Symphonia teaches that the church and the state cooperate in the church's mission. It goes back to Emperor Justinian's time, considered a golden age for Orthodoxy. It is the polar opposite of the Western idea, in which one of the functions of the Roman Catholic Church was to act as a bulwark against the political vacillations of the emperors, who all too often promoted heresy through the power of the state. It differs even more from the idea of separation of church and state which emerged after the American Revolution.
In the 20th century, the Greek Orthodox Church at times intervened to protect the independence of Eastern European Orthodox congregations whose religious freedom it saw as threatened by Soviet policy, which was militantly atheistic. Today it is doing something similar with regard to Russian incursions of Ukraine. As early as 2019 it recognized the Ukrainian metropolitan bishop as being independent from Moscow. A Synod in Athens in October of that year officially affirmed:
...the canonical right of the Ecumenical Patriarchate to grant
autocephaly [independent Petrine authority], as well as the privilege of the primate of the Greek Church to further pursue the
issue of the recognition of the Church of Ukraine.
The Russian Orthodox Church, on the other hand, sees Russian leader Vladimir Putin as a protector of Orthodoxy against western secularism. Under Putin's regime, it formed close ties to the Kremlin and supported the Russian invasion of Ukraine. It has publicly split with the Greek Orthodox patriarch by appealing to the historical unity of the Ukrainian and other national church under the Metropolitan of Moscow:
The Russian Orthodox Church and its first see – the Kiev Metropolia –
constituted a whole for centuries, despite various political and historical
tribulations which at times shattered the unity of the Russian Church.
The Patriarchate of Constantinople, that originally had jurisdiction
over the Russian Orthodox Church, consistently defended its unity
until the middle of the 15th century, as was later reflected in the
title of the Metropolitans of Kiev – “of All Russia.” And even after
the Primatial see had been transferred from Kiev to Vladimir and then
to Moscow, the Metropolitans of All Russia were stilled called
Metropolitans of Kiev.
Both positions are justified under the doctrine of Symphonia. The Greek Orthodox can point to the many cases where Orthodox monks and bishops played a prophetic role by standing up to Imperial authority when it erred. The Russian Church can insist that its policy is not mere nationalism, but strongly rooted in Orthodox principles of working with the state to further the Christian cause.