Just to make it perfectly clear what ateism is:
BTW: This is fact, not my opinion.
Secular/Nonreligious/Agnostic/Atheist: This is a highly disparate group and not a single religion. Although atheists are a small subset of this grouping, this category is not synonymous with atheism. Atheists actually make up less than one-tenth of one percent of the population in many countries where large numbers claim no religious preference, such as the United States (7.5% nonreligious) and Australia (15% nonreligious).
One portion of this broad grouping includes those who are best described as "nonreligious," i.e., those who are essentially passive with regards to religion, generally affirming neither belief nor disbelief. They may be neither contemplative about philosophy and spirituality nor involved in a religious/faith/philosophical community. Although a certain percentage of people in many countries classify themselves as nonreligious in surveys, there are few data indicating how many of these fit the passive "nonreligious" criteria described above, versus those who actually do contemplate such matters, but simply have their own personal philosophy and no stated affiliation with an organized religion.
For the purposes of this list, this grouping also includes more proactive or well-defined philosophies such as secular humanism, atheism, agnosticism, deism, pantheism, freethought, etc., most of which can be classified as religions in the sociological sense, albeit secular religions.
The "Secular/Nonreligious/etc." category is probably the most speculative estimate in this list, as this segment of society is difficult to count. The vast majority in this grouping are not aligned with any kind of membership organization. Most figures come from census and survey data, which most countries conduct only infrequently.
The highest figure we have for "Nonreligious" is 20% of the world population, or about 1.2 billion: "Over 20 percent of the world's population does not claim any allegiance to a religion. Most are agnostics. Others are atheists, who deny the existence of God." (O'Brien, Joanne & Martin Palmer. The State of Religion Atlas. Simon & Schuster: New York (1993). Pg 41.) But such a high figure is difficult to support with current country-by-country statistics, and perhaps reflects Communist-era official government statistics. Most current estimates of the world number of secular/nonreligious/agnostic/atheist/etc. are between 800 and 1 billion.
Estimates for atheism alone range from 200 to 240 million. But these come primarily from China and former Soviet Union nations (especially Russia). Prior to Communist takeovers of these regions and government attempts to eradicate religion, both places had very high levels of affiliation with organized religions (especially Islam, Christianity, Buddhism and Taoism), as well as high levels of participation in and belief in traditional local traditions such as shamanism, ancestor ceremonies, spiritism, etc. Since the fall of Communism in former Soviet nations and the relaxation of anti-religious policies in China, observed religious affiliation and activity has increased dramatically, especially in Christianity, Buddhism, and Islam.
China probably does have the largest number of actual atheists of any country in the world and many Russians clearly remain atheists. But at this point, it is impossible to accurately determine how many of those classified as atheists or nonreligious during Communist-era USSR and by the current Chinese government are actually atheists according to their personal beliefs, and how many are unregistered religious adherents or participants in less-organized traditional systems that are oriented around ancestors, animism, shamanism, etc. Many people are unaware, for instance, that China has one of the largest, most active Christian communities in the world, and that in many former Soviet nations religions such as shamanism, Islam and Russian Orthodoxy remained even while official government reports announced the elimination of religion in these regions.
In the Western world, Europe is by far the place with the most self-avowed nonreligious, atheists and agnostics, with the nonreligious proportion of the population particularly high in Scandinavia. The Encyclopedia Britannica reports approximately 41 million atheists in Europe. The self-described nonreligious segment of society in Australia and New Zealand is also high, at around 15%. In Australia less than a tenth of one percent described themselves as atheists in the latest national census (1996). In the U.S. about 7.5% of the population describe themselves as nonreligious, 0.7% describe themselves as agnostic, and a smaller number describe themselves as atheist (Kosmin, National Survey of Religious Identification, 1990).
All those who profess religious belief are not necessarily registered members of a church or denomination, but in the U.S. the majority of professed Christians and adherents of other religions are also officially affiliated with an organization. The majority of agnostics, atheists and of course nonreligious are not members of an organization associated with their position.
It may also be noted that the estimated figures presented in this particular "Major Religions" summary list are based on self-identification. Among all groups there exists a proportion (sometimes significant and sometimes small) which are only nominal adherents. This segment may identify themselves as members of a certain religion and accept the religion as their primary philosophical system, yet not actively practice the religion in the normative sense. This segment may be thought of as being functionally nonreligious or "secularized," but this segment is not what is meant by the "nonreligious" category on this Major Religious list. Accurate estimates of the size of this group are difficult to obtain because national government censuses only ask about preferred affiliation, not about religious practice. There are data available from non-census sampling surveys that ask about practice and belief, but these are usually limited in scope to narrow questions such as church attendance, and do not entirely reveal the proportion of society which is non-attending, but nevertheless privately practicing and/or believing. In many countries (Germany is a good example) there is also segment of the population which is counted as adherents of a religion, but which do not personally profess belief in that religion. (Adherents.com has some such data in its main list under "attendance" and under "poll".) See also: Top 10 Countries with Most Atheists.
The use of the term "nonreligious" or "secular" here refers to belief or participation in systems which are not traditionally labeled "religions." Of course, in the absence of traditional religions, society exhibits the same behavioral, social and psychological phenomena associated with religious cultures, but in association with secular, political, ethnic, commercial or other systems. Marxism and Maoism, for instance, had their scriptures, authority, symbolism, liturgy, clergy, prophets, proselyting, etc. Sports, art, patriotism, music, drugs, mass media and social causes have all been observed to fulfill roles similar to religion in the lives of individuals -- capturing the imagination and serving as a source of values, beliefs and social interaction. In a broader sense, sociologists point out that there are no truly "secular societies," and that the word "nonreligious" is a misnomer. Sociologically speaking, "nonreligious" people are simply those who derive their worldview and value system primarily from alternative, secular, cultural or otherwise nonrevealed systems ("religions") rather than traditional religious systems. Like traditional religions, secular systems (such as Communism, Platonism, Freudian psychology, Nazism, pantheism, atheism, nationalism, etc.) typically have favored spokespeople and typically claim to present a universally valid and applicable Truth. Like traditional religions, secular systems are subject to both rapid and gradual changes in popularity, modification, and extinction.