State atheism is the incorporation of hard atheism or non-theism into political regimes. It is considered the opposite of theocracy and may also refer to large-scale secularization attempts by governments. To some extent, it is a religion-state relationship that is usually ideologically linked to irreligion and the promotion of irreligion or atheism. State atheism may refer to a government's promotion of anti-clericalism, which opposes religious institutional power and influence in all aspects of public and political life, including the involvement of religion in the everyday life of the citizen. In some instances, religious symbols and public practices that were once held by religions were replaced with secularized versions of them. State atheism in these cases is considered as not being politically neutral toward religion, and therefore it is often considered non-secular.

The majority of communist states followed similar policies from 1917 onwards. The Soviet Union (1922–1991) had a long history of state atheism, whereby those who were seeking social success generally had to profess atheism and stay away from places of worship; this trend became especially militant during the middle of the Stalinist era, which lasted from 1929 to 1953. In Eastern Europe, countries like Bulgaria, East Germany and Czechoslovakia experienced strong state atheism policies. The Soviet Union attempted to suppress public religious expression over wide areas of its influence, including places such as Central Asia. Currently, China, North Korea, and Vietnam, are officially atheist.

Cuba was an atheist state until 2019, when a change in its constitution declared it a secular state.

In contrast, a secular state officially purports to be neutral in matters of religion; it does not support religion, nor does it support irreligion. In a review of 35 European states in 1980, 5 states were considered "secular" in the sense of religious neutrality, 9 considered "atheistic", and 21 states considered "religious".

A communist state is a state with a form of government which is characterized by the one-party rule or the dominant-party rule of a communist party which professes allegiance to a Leninist or Marxist–Leninist communist ideology as the guiding principle of the state. The founder and primary theorist of Marxism, the 19th-century German thinker Karl Marx, had an ambivalent attitude toward religion, which he viewed as "the opium of the people", simultaneously "the sigh of" and a source of moral agency of the "oppressed creature" against their suffering. To Marx, religion was not the ideological expression of those in power, and he did not see it needing abolishing. Instead, he saw the communist state as creating conditions where the consolation provided by religion would not be needed. In the Marxist–Leninist interpretation of Marxist theory, developed primarily by Russian revolutionary Vladimir Lenin, atheism emanates from its dialectical materialism and tries to explain and criticize religion.

Lenin states:

Religion is the opium of the people—this dictum by Marx is the corner-stone of the whole Marxist outlook on religion. Marxism has always regarded all modern religions and churches, and each and every religious organisation, as instruments of bourgeois reaction that serve to defend exploitation and to befuddle the working class.

Although Marx and Lenin were both atheists, several religious communist groups exist, including Christian communists.