The claim is essentially correct -- tens of millions of Orthodox Christians died of political violence in the 20th century. However, everything that Mr. Serfes tries to imply by making that claim is basically wrong.
First, it's hard to be more precise about the claim because Mr. Serfes' essay loaded with so much hyperbole that his claim is almost nonsensical. The specific claim is that "up to 50 million Orthodox Christians have perished in the first eight decades of the twentieth century." He is actually asserting an upper limit, though his thesis is based on conveying the impression that this is a large number of people -- which requires a lower limit instead. Serfes basically accounts for 15 million deaths (or so), and then bumps the number up by a few tens of millions by appealing to the general political turmoil of Eastern Europe during the 20th century.
While this gives him a nice big headline figure, it undermines his thesis by demonstrating that there is nothing special about the suffering of the Orthodox Christian community -- Orthodox Christians simply lived in a region of the world that was in constant political turmoil since roughly the 1880s, and therefore a lot of them died violent deaths, just like a lot of people in other parts of the world. I do not mean to trivialize their suffering, I am simply providing context for the facts that Serfes cites in his political diatribe disguised as a memorial.
An additional complication in evaluating the claim arises from the unclear and possibly inconsistent definition of "orthodox Christian" that Serfes uses. As suggested in the question, it appears that he treats the term as synonymous with "person of a traditionally Orthodox Christian nationality". This is consistent with how the term "Jew" is used with respect to the Nazi's genocide towards the Jewish people, since the Nazi's defined "Jew" based on genetic heritage and not any profession of faith. Furthermore, it may be consistent with some modern American notions of religious discrimination, since such discrimination is driven by the perception of religious identity, regardless of actual religious belief. It's possible (and likely) that membership in a historically Orthodox national group was taken as evidence of Orthodox Christian faith by the attackers. However, in the cases that Serfes cites, I see little indication that the attacks were driven by religious animosity rather than nationalist or racist animosity -- or simply the desire to dominate others.
In addition, it is unclear what Serfes includes under his umbrella term "Christian Orthodox". From my brief reading, it seems that the Eastern Orthodox communion (Russia, Greece, Ukraine) and the Oriental Orthodox communion (Armenia, Ethiopia) have some substantial theological differences; institutionally, they diverged prior to the "Great" Catholic/Eastern schism.
Serfes also includes the Assyrian and Ethiopian Churches among his list of victims. I'm not sure why he lumps these branches together -- is it simply their geographical clustering? Perhaps he considers the Great Schism to be the fundamental division in Christianity (with the Eastern Orthodox church being affiliated with the Oriental and Assyrian churches). Or perhaps -- to acknowledge his thesis of Orthodox persecution -- he suspects that Western Christians view the "pre-Roman Catholic" churches to be somehow primitive.
A final problem is that Serfes seems to exclude Orthodox vs. Orthodox violence from his tally. This becomes a serious complication when we aren't sure who to count as Orthodox -- I get the impression that Russian Communists are to be counted as "Orthodox" when they are the victims of political violence, but not the perpetrators.
In general, I will be tallying the deaths of people from Orthodox Christian nationalities, and noting the relevant complications as they arise. I doubt that it is possible to provide a more precise accounting of "Orthodox Christian" deaths since the Orthodox church was the state-sponsored religion for many of these nationalities.
Serfes has essentially summed up the death counts from a variety of unconnected events, which I will list below.
1) The Armenian Genocide (1915-1923): up to 1.5 million people. This doesn't contribute much to the 50 million deaths, so I'm not going to quibble over the numbers and I'll just take the value from the people who are trying to raise awareness of the genocide (Serfes claimed 1.8 million). However, this has a strong psychological impact because it was a genocide attempt (a Holocaust, if you will). Furthermore, the modern Turkish state still favors Islam (sometimes at the expense of Orthodox Christians) and persecutes those who raise awareness of the genocide, so this supports Serfes' contention that Orthodox Christians are an marginalized minority.
However, it also undermines his assertion that the USA and Western governments are insensitive to the plight of Orthodox Christian nationalities, since Turkey's NATO allies regularly risk diplomatic rifts by bringing up the issue of the Armenian Genocide, just as they condemned it as it happened.
Another note on this topic is that the Ottoman/Turkish governments that perpetrated the genocide were not Islamist (as implied in the question), they were secular Nationalist (even militantly secular).
2) The Balkans, Greece, and the fall of the Ottoman Empire (1912-1922): Serfes claims 1.75 million Greeks "murdered by Turkish persecutions", with the massacre of Smyrna (1922) as the touchstone of this violence. Again, the numbers are a small portion of the total, so I'm not going to bother with the details. While Serfes limits his count to WWI period (and the final collapse of the Ottoman Empire), this violence seems to be intimately linked with the 19th century's wars of Independence for the Greek(1821) and Balkan Christian nations, along with the Russian expansion into Ottoman territory.
The massacre of Smyrna seems to be the finale to an almost continuous period of violence ranging from the Balkan Wars (1912-1913), through WWI (1914-1918), and the Greko-Turkish war (1919-1922). While these wars mainly saw Orthodox Christian Nations allied against the Ottoman Empire (with Western powers aiding either side as suited them), Bulgaria often fought against other Orthodox Christian nations (in the Second Balkan War and WWI).
Cumulative total: up to 2.25 million
3) World War I (1914-1918): Serfes does not mention WWI directly, only indirectly in his summary statement mentioning "warfare". There were probably about 5 million deaths among residents of traditionally Orthodox nations (Russia, Romania, Serbia, Greece, Bulgaria). Romania and Serbia had a particularly large number of deaths as a portion of the population.
Orthodox Christian nationals made up 1/3 of the total deaths.
Cumulative total: up to 7.25 million
4) Communist revolutions(1918 and later): Serfes clearly dislikes the Bolsheviks for their anti-religious policies. Perhaps one or two million deaths can be attributed to these wars.
"First in Russia and Ukraine, then in Eastern Europe, in Greece during its civil war (1945-49), and in Ethiopia, the Orthodox Church was the principle target for attach, subversion, or destruction."
I don't know that a specific number of deaths can be attributed to specifically anti-religious activities, especially since the church was tightly aligned with the monarchy during these wars.
Cumulative total: up to 9.25 million
4) The Ukranian Famine (1932-1933): This is perhaps the cornerstone of Serfes' tally. He reports 7-12 million deaths due to hostility from the Soviet Union; the encyclopedia Brittanica reports 4-5 million. The Rutgers Center for the Study of Genocide and Human Rights reports 10 million. The Ukrainian law regarding the "Holodomor" does not provide a number, but a website specifically dedicated to the Holodomor tallies 5.5 million (4 million from starvation; 1.5 million from dekulakization campaigns). In general, I do not see serious estimates over 10 million, so that's the value I'll use.
Cumulative total: up to 19.25 million
5) World War II (1939-1945): Again, Serfes hardly mentions WWII, but this surly is the bulk of the 50 million deaths that he counts. The traditionally orthodox nations suffered up to 30 million deaths during the war, almost all of them being from the Soviet Union, at the hands of Nazi Germany and their fascist allies -- including Bulgeria and Romania. Other sources gave slightly lower estimates.
Overall, deaths among Orthodox Christian Nationalities accounted for about 1/3 of the deaths in WWII (other major contributors were China, Poland, and Germany)
Cumulative total: up to 49.5 million
But does it mean anything?
When this question was posted, I objected that it was a bad question and little more than political propaganda. This tally is from a number of essentially unrelated events, and grouping them together only makes sense in the context of some theory that links them. What are those possible theories?
1) Orthodox Christians are a community, and members of a community should recognize the suffering that occurred within it. I could respect this theory, but unfortunately this does not seem to be the theory that Serfes is promoting.
2) These deaths are part of a larger trend of oppression of Orthodox Christians. This appears to be Serfes' agenda, and it is dangerous bullshit. This theory has two components: first, that Orthodox Christians were treated especially brutally by other communities; second, that other communities continue to minimize and ignore the suffering done to Orthodox community. I say that this theory is bullshit because neither of those claims are true. I say that the theory is dangerous because Serfes' presentation comes very close to minimizing the suffering of other communities (e.g. Jews) and excusing abuses committed by Orthodox Christians.
Orthodox Christians were not singled out for abuse (mostly)
Under the USSR
The primary culprit in these deaths is the Soviet Union (through enslavement, political terrorism, wars of aggression, and general economic/military incompetence). The Soviet government exploited and terrorized all populations in its territory. It attacked any group that offered an alternative to the USSR -- whether Orthodox, Catholic, Muslim, or secular. It did not single out Orthodox Christians; if that group made up the largest portion of its victims, its simply because the USSR emerged from an Orthodox Christian society.
Serfes' thesis of Orthodox oppression relies on the perception that all of these hardships came at the hands of "outsiders". The Soviet Union should not be viewed as an outside group oppressing a Eastern Orthodox nation -- the Bolshivik movement was a home-grown Russian political movement. Serfes wrote his essay before Putin came to power, but I wonder what he thinks of Putin's redefining Russian identity in a way that embraces the Orthodox Church at the same time it claims the glory of the USSR (especially with respect to Ukraine). Even Western communists have rejected the Soviet Union for over half a century. The current crisis in Ukraine reveals that many Ukrainians see their oppression under the USSR as being a nationalist issue as much as a ideological/religious one.
Finally, there is not enough space here to list all of the groups that were targeted by the Soviet state (many different nationalities and ideologies). One particularly relevant group is the Crimean Tartars, a Muslim population that suffered the Holodomor along with their Orthodox Christian neighbors. After WWII, the Tartars suffered mass deportation to Siberia, during which a huge portion of their population died.
The secondary culprit in these deaths is Nazi Germany, through its invasion of Eastern Europe and genocidal policies towards various populations. The policy of "Lebensraum" (living space for Aryans) was particularly destructive towards the people of Eastern Europe. The Nazis had a special disdain for the Slavic people to their east, and were more tolerant of the Germanic and Latin peoples to their west (though the French were effectively enslaved like everyone else). The hostility towards Slavs manifested as genocidal plans that would have substantially reduced the eastern populations -- particularly the Catholic Poles and Lithuanians. The Orthodox Christian nationalities (particularly Ukrainians) were definitely harmed by these policies, but they were not singled out. The Nazis even had two Orthodox nations (with Orthodox Christian leaders) as allies -- Romania and Bulgaria.
This is the one situation in Europe where it appears that Christians were specifically marginalized and massacred. Turkey obviously still has a lot of problems with its institutions. I think Serfes exaggerates the influence that the USA has over Turkey. Sill, Christian nationalities were not the only victims of Turkish nationalism -- the Kurds have a long history of conflict with the Turkish state.
Orthodox Christians as Assailants
The last set of events to consider are those where Orthodox Christians were the assailants. My purpose in bringing this up is to show that this region of the world was generally unstable and violent during the last century. I don't mean to imply that the Orthodox Christians were particularly aggressive or that they (as a community) deserved any of the suffering that Serfes listed.
First, there is the issue of whether to count home-grown communist movements as "Orthodox Christian". I've discussed that above and will leave communist aggression out of this count. I'll also leave out anti-Semitism. Here are the events I can think of:
- Balkan wars: massacres of Albanian Muslims and exile of Ottoman muslims.
- Russo-Ottoman wars: Ethnic cleansing of areas conquered by Russia.
- Breakup of Yugoslavia: Ethnic cleansing by Serbs in Croatia, Bosnia, and Kosovo.
- Russian suppression of Chechen Independence and the destruction of Grozny.
This is what makes Serfes' diatribe particularly dangerous. Orthodox Christians are the dominant community in many countries (such as Russia), which makes this narrative of oppression both absurd and dangerous. Serfes even goes as far as to suggest that "Christians" should always give the benefit of the doubt to other Christians (e.g. Serbia) in their conflicts with non-Christians, and likewise to suggest that the American government should not press Christian nations (such as Russia) to increase religious freedom until non-Christian allies (e.g. Turkey) have instituted full religious freedom. This is a recipe for tyranny and a continuation of these ancient blood feuds.
The Ukrainian famine is a major item in this list, and I want to provide a little more context. First, other sources disagree with Serfes contention that the famine of 1932-33 was limited to Ukraine. The Brittanica article estimates that 1/3 of the victims were outside of Ukraine (though it's unclear if that includes Crimea, which was part of Russia at the time). Wikipedia has more information.
I'll also note the similarities between the Ukranian famine and the Great Irish Famine and the Great Chinese Famine. Briefly, all involved an element of enslavement and exploitation, where a food producing country continued to export food while its residents suffered starvation and were prevented from growing food for themselves. Like the Ukrainian famine, the Great Irish Famine also involved a power relationship between nations (England/Ireland; Russia/Ukraine). The Chinese Famine is similar to the Ukrainian famine in its connection to Stalinist collectivization and the use of starvation to destroy traditionalist communities and punish political opponents.
Finally, the Holodomor is only comparable to the Nazi campaign against the Jews in that both were horrible. Most notably, the USSR sought to break the resistance of the Ukrainian community, not to kill every single one of them. In the context of Serfes' thesis, this comparison of Orthodox community's trauma to the Nazi genocide against the Jews implies that the only reason for popular concern with anti-Semitism is the political influence of Jews. I hope that's not what he means to imply, since the history of Jewish marginalization and persecution in Western society is very different from the trauma experienced by Orthodox Christians in Western society.