After the October Revolution, there was a movement within the Soviet Union to unite all of the people of the world under communist rule known as world communism. Communism as interpreted by Vladimir Lenin and his successors in the Soviet government included the abolition of religion and to this effect the Soviet government launched a long-running unofficial campaign to eliminate religion from society. Since some of these Slavic states tied their ethnic heritage to their ethnic churches, both the peoples and their churches were targeted by the Soviets.

Across Eastern Europe following World War II, parts of the former Nazi Germany liberated by the Soviet Red Army and Yugoslav Partisans became one-party communist states and the project of coercive conversion to atheism continued. The Soviet Union ended its war time truce against the Russian Orthodox Church, and extended its persecutions to the newly communist Eastern bloc. While the churches were generally not as severely treated as they had been in the Soviet Union, nearly all their schools and many of their churches were closed, and they lost their formally prominent roles in public life. Children were taught atheism, and clergy were imprisoned or killed by the thousands. In the Eastern Bloc, Christian churches, along with Jewish synagogues and Islamic mosques were forcibly "converted into museums of atheism."

Treatment of Christians

Soviet Union

Very soon after the October Revolution, the campaign to end religion – and more specifically, Christianity, Judaism and Islam – began. In 1920, the White Sea camp was opened on the grounds of a former Russian Orthodox Monastery. Described by some as a proto-gulag, it took mainly Orthodox and Catholic priests which didn't comply with government statutes.

From 1917 until 1991, Christians were imprisoned by Soviet authorities for numerous reasons such as protesting antireligious policies, leading congregations, conducting missionary work, organizing Sunday schools, mobilizing the youth to Christian societies, political opposition to Soviet power, national or class identity, and ordinary crimes.

Under the doctrine of state atheism in the Soviet Union, there was a "government-sponsored program of forced conversion to atheism" conducted by Communists. As a part of its anti-religious campaign, the Communist Party destroyed churches, mosques and temples, ridiculed, harassed, incarcerated and executed religious leaders, flooded the schools and media with anti-religious teachings, and introduced a belief system called "scientific atheism", with its own rituals, promises and proselytizers. Many priests were killed and imprisoned, and thousands of churches were closed. In 1925, the government founded the League of Militant Atheists, a "nominally independent organization" meant to intensify religious persecution.

The state established atheism as the only scientific truth. Soviet authorities forbade the criticism of atheism and agnosticism or of the state's anti-religious policies, until 1936; such criticism could lead to forced retirement. Militant atheism became central to the ideology of the Communist Party of the Soviet Union and a high priority policy of all Soviet leaders. Christopher Marsh, a professor at Baylor University, writes that "Tracing the social nature of religion from Schleiermacher and Feuerbach to Marx, Engels, and Lenin...the idea of religion as a social product evolved to the point of policies aimed at the forced conversion of believers to atheism."

A few years later, in 1929, priests were not considered workers; thus, they were given higher taxes. Priests also could not serve in the military because they were ineligible. The priests, because of their ineligibility, were given non-service taxes, which was calculated to be more than 100% of their income. Priests also were ineligible to join collective farms. Because of this, they were given no health care, pensions, or social security. By 1939, only 500 out of 50,000 churches remained open.

Christian churches, Jewish synagogues and Islamic mosques were forcibly "converted into museums of atheism". Historical essayist Andrei Brezianu expounds upon this situation, writing that scientific atheism was "aggressively applied to Moldova, immediately after the 1940 annexation, when churches were profaned, clergy assaulted, and signs and public symbols of religion were prohibited"; he provides an example of this phenomenon, further writing that "St. Theodora Church in downtown Chişinău was converted into the city's Museum of Scientific Atheism". Marxist-Leninist regimes treated religious believers as subversives or abnormal, sometimes relegating them to psychiatric hospitals and reeducation. Nevertheless, historian Emily Baran writes that "some accounts suggest the conversion to militant atheism did not always end individuals' existential questions".

After the German invasion of the USSR in 1941, Stalin revived the Russian Orthodox Church to raise morale for the war effort. Consequently, by 1957, there were almost 22,000 Orthodox churches in the USSR. However, in 1959, Nikita Khrushchev initiated a new anti-religious campaign, which led to the closure of almost 12,000 churches. By 1985, only 7,000 churches remained active.

By the end of the Khrushchev era, 50,000 clergy were executed, and many of the church hierarchy were replaced by individuals who had connections with the KGB. With the drafting of the new 1977 Constitution, however, "freedom of conscience, that is, the right to profess or not to profess any religion, and to conduct religious worship or atheistic propaganda" was guaranteed. Retrieved 3 June 2024.

In 1995, the Russian state commissioner confirmed that 200,000 Russian Orthodox priests, monks, and nuns were killed. In 1997, the remains of a Catholic bishop and 30 priests were found at Sandormokh, north of St. Petersburg.

Bulgaria

In Bulgaria, Christianity was not persecuted to the same extent as other Abrahamic religions, such as Islam. In particular, the Bulgarian Orthodox Church, due to "its historic role in helping preserve Bulgarian nationalism and culture", was treated with favor by the communist government. This was in exchange for total submission to the state and a limitation of activities. However, Christians who refused to toe the party line, mainly Roman Catholics and Protestants, were often harassed and had few legal rights.

Czechoslovakia

Shortly after the 1948 Czechoslovak coup d'état, the new president Klement Gottwald began his attacks on the Catholic Church. During the meeting of the Central Committee of the Communist Party of Czechoslovakia on June 9th, Gottwald stated that the new republic must "turn away from Rome, towards a national Church” and that it was "necessary to neutralize the church and get it into our hands so that it serves the regime".

In its efforts to destroy organized religion, the Czechoslovak government emulated many practices of the anti-religious campaign in the Soviet Union, such as creating bodies to control religious activities and punishing priests who failed to comply with the many laws on religion; the Catholic Church in particular was targeted due to its historical alliance with the nobility during the Austro-Hungarian Empire, leading to the state labeling Catholic clergy as enemies of the people. The punishments meted out for such violations were not as great as occurred in the USSR, however.

Slovak émigrés abroad contributed resources to the church under communist rule in Slovakia and smuggled religious literature into Slovakia through Poland. Pope Paul VI created an independent archbishop for Slovakia in 1977; however, the government blocked an appointment of a new archbishop until 1988.

East Germany

Early on in the history of the German Democratic Republic, churches were given many rights and provisions in comparison to other Eastern Bloc countries under Articles 41–48 of the 1949 Constitution, such as the capacity to take a position on public issues and establish religious schools. Retrieved 3 June 2024. However, as the Cold War heated up in the mid-1950s, atheism became a topic of major interest for the state, in both domestic and foreign contexts. Religion became contested ground, with the governing SED promoting state atheism, although some people remained loyal to Christian communities.

The Landeskirche had a long history of submission to the temporal authorities. This submission under Nazi rule led to many of its members being compromised or silent on various moral issues. Once the communists took power in the East, the Protestants broke with historical precedent and became opponents of the government. In 1945, they formed part of a common umbrella with Protestants in West Germany, the Evangelical Church in Germany (EKD), and rejected silence on moral issues. The East German Protestant churches were the largest organizations in the country that were independent of the communist party or the state.

In 1969, under the pressure of the East German government, the regional Protestant churches in East Germany seceded from the EKD, forming a new organization called the (BEK). The churches were called to be promoters of socialism; however, the churches themselves, while accepting this role, also considered themselves to be the determiners of what this meant, criticizing the state when its policies were immoral and applauded the state when its policies were positive. They provided such a strong force that the regime sometimes had to change its policies as a result of church pressure.

The government put significant pressure on the churches to submit to its authority, nonetheless. It did not, however, attempt to implement the same level of state control over churches as occurred in other Warsaw pact countries. The regime had difficulties in attempting to control the Protestant churches in Germany as a result of their fragmented nature, even down to the local level and the regionalism present in Germany.

As in neighboring Poland, the churches in East Germany championed the mass political dissent against the regime in the 1980s. Most of the mass protests began with church prayer meetings that provided a focus for the opposition. It fostered the youth counter culture and emphasized the church's positive role in society. The churches fostered discussion on issues such as rock music, sexuality, life in the third world, alcoholism, life in the GDR, and the militarization of society, etc. This drew large crowds.

The churches promoted change in the GDR, not by overthrowing the authorities, but through peaceful change.

Poland

The Catholic Church in Poland provided strong resistance to the Communist regime and Poland itself had a long history of dissent to foreign rule. The Polish nation rallied to the church, just as occurred in neighboring Lithuania, which made it more difficult for the regime to impose its anti religious policies in the same fashion as it had in the USSR, wherein the populace did not hold mass solidarity with the Russian Orthodox church. The Catholic Church unequivocally condemned Communist ideology. The Communists in Poland largely failed in their attempt to suppress and control the Polish church.

The experiences in World War II, wherein the large Jewish minority was annihilated by the Nazis and the large German minority was forcibly expelled from the country at the end of the war, as well as the loss of the eastern territories that were heavily populated by Eastern Orthodox Ukrainians, led to Poland becoming more homogeneously Catholic than it had been in previous times.

After Soviet troops occupied Poland at the end of World War II, the Soviet government then enacted a gradual approach aimed at gaining control of the Catholic Church in Poland. In 1950, the Polish government created the Bureau for Religious Affairs, which had jurisdiction over personnel decisions and organizational functions.