Regardless of its theological status, Communism is classified as a religion in the sociological sense.
Note that this list only includes countries which currently have Communist governments. Most countries have an organized Communist Party, but their statistics are not included here.
Total Number of Party Members
Source of population estimates, calculated for July 2006: CIA World Factbook, http://www.cia.gov/cia/publications/factbook/rankorder/2119rank.html; viewed 20 June 2006
Mark McDonald. "Vietnam's Communist party struggles to attract youths" in San Jose Mercury News, 6 February 2000:
But very few Vietnamese -- only about 3 percent of the population -- are actually members of the Communist Party. Total membership today stands at about 2.4 million, with the bulk of the members being farmers, government careerists, military officers and conscripts, and workers in state-owned agencies and factories.
Unlike the Communist organizations of Eastern Europe and the USSR, the Chinese Communist party (CCP) was able to stem the tide of democratic protest in the late 1980s. Founded in 1921, it is the largest Communist party in the world, with an estimated membership of more than 58 million in the late 1990s.
The final argument is that Cuba is socialist because it is ruled by the "direct democracy" of so called "mass organisations". On paper the Cuban Communist Party is a mass organisation, with 700,000 members, and 600,000 in the Young Communists (UJC). But even sympathisers recognise that it is not mainly composed of workers from the factories and the fields, but of plant managers and bureaucrats. It is not a party in the real sense. Its top leaders are not subject to any kind of re-election or recall by the ranks. It does not compete with any other parties. They are illegal in Cuba. Who joins the CP depends on the party's Secretariat, which is itself subject to the Politburo who are elected by and accountable to no one. If you read the official propaganda, it says that the unified Cuban nation needs only one party to maintain its cohesion.