The term atheism is formed of the Greek prefix a- (meaning "without" or "not") and the Greek-derived theism, meaning a belief in a god or gods. The literal meaning of the term is therefore without a belief in a god or gods, making any person who does not believe in the existence of a god or gods an atheist -- including both those who assert that no gods exists and those who, for whatever reason, do not make any assertion regarding the existence of gods. By this definition, all babies are atheists, since they have never heard of God. In modern usage as reflected in most dictionaries, atheism is the assertion that no gods exists, or that the existence of gods has infinitesimal probability. Atheism is not synonymous with irreligion; the idea of an eternal non-created universe is an important concept in some religions and as such it is possible to be very religious and very atheistic.
The term agnosticism (coined by T.H. Huxley in 1869) describes a form of philosophical scepticism in which the existence of gods is considered undecidable or in which inquiry into the existence is considered unproductive.
Some writers, especially online, use the term "strong atheism" (also, "explicit", or "hard" atheism) for the denial of the existence of gods, and "weak atheism" (also, "implicit", or "soft" atheism) for mere lack of belief, without the active denial of gods. In the freethought tradition, analogous terms are "positive" and " negative" atheism. Within this scheme, agnosticism represents a rationale for weak atheism. However, none of these terms are in common use.
The Encyclopędia Britannica estimates that about 2.5% of the world's population classifies itself as atheist. People are considerably more likely, about 12.8%, to describe themselves as "non-religious". Atheism is somewhat more prevalent in Europe and Russia than in the United States and developing countries. For instance, according to a 2003 poll, 33% of French adults say that "atheist" defines their position on religion rather or very well.
Atheists who openly express their views have often been mistreated, ostracized, discriminated against, or, in some countries, killed. For this reason, it is possible that atheism is more prevalent than polls suggest, as people may be reluctant to express their true views. Those who hold theistic views often consider that those without a belief in God must be immoral or untrustworthy -- unfit as members of society. The scriptures of Abrahamic and some other religions contain denunciations of non-believers. In Europe's Middle Ages, atheism was regarded as immoral, sometimes even criminal; atheists could be sentenced to death by burning, especially in countries where the Inquisition was active. Even though Protestantism suffered on its own from discrimination and persecution by the Roman Catholic Church, John Calvin was also in favor of burning atheists and heretics.
On the other hand, atheism has at times been the official stance of some countries, such as the former Soviet Union, the former Eastern Bloc, and the People's Republic of China. Karl Marx, an atheist, wrote that religion is "the opiate of the masses" and is harmful to society, as it encourages workers to accept the current social order. Marxist doctrine aside, Communist states may have found it expedient to discourage religion so as to weaken centers of opposition to the state. In the Soviet Union and the People's Republic of China, churches that submitted to state control were tolerated.
Every military buildup in the United States since WWII has been accompanied by frequent use of the canard "There are no atheists in foxholes." During the Cold War, the fact that the communist antagonists of the United States were officially atheistic ("godless communists") contributed to the view that atheists were unreliable and unpatriotic. As recently as the 1987 presidential campaign in the officially secular United States, George H. W. Bush said , "I don't know that atheists should be regarded as citizens, nor should they be regarded as patriotic. This is one nation under God." Similar statements were made during the controversy surrounding the inclusion of the phrase "under God" in the American Pledge of Allegiance, words which were added to the pledge early in the Cold War period.
Nothwithstanding Cold War attitudes, atheists are legally protected from discrimination in the United States. And they have been among the strongest advocates of the legal separation of church and state. American courts have regularly, if controversially, interpreted the constitutional requirement for separation of church and state as protecting the freedoms of non-believers, as well as prohibiting the establishment of any particular religion. The legal situation is often summed up by the phrase: "Freedom of religion also means freedom FROM religion."