In a strict sense, atheism denotes the belief that god doesn't exist. In a wider sense atheism is equated with the absence of belief in any and all religious ideas, beings (gods, angels, spirits, etc.), and holy texts. The wider conception of atheism is often called "irreligiousness" or "non-religiousness" in social sciences. "Agnosticism" is a concept related to atheism and irreligiousness, and it represents the belief that the existence of sacred, supernatural, and religious things can not be verified empirically. Sociological studies of atheism developed several typologies for the classification of different forms of atheism.  

            Explicit and Implicit, and Weak and Strong Atheists

The first type of division is into people who simply lack a belief in god and into those who explicitly believe that god doesn't exist. The first group of atheists is called "implicit atheists". Examples are: a) people who have never heard of the concept of god, like those who are living in cultures whose religious ideas don't include the belief in god; b) people who are undecided on the issue of whether they believe or not believe in the existence of god; c) individuals who truly don't care whether god does or doesn't exist. The other group of atheists is "explicit atheists".  

The second division of atheists is into "weak atheists" (also sometimes labeled as "soft" or "negative" atheists) and "strong atheists" (also called "hard" and "positive" atheists). The first group includes all types of implicit atheists but it also includes all individuals who explicitly don't believe in the existence of god, but, at the same time, would not agree with the statement "I believe that there is no god".  Their lack of belief in the existence of god should not be confused with their unwillingness to say that there are completely certain that god does not exist. Strong atheists, on the other hand, are those who are certain and truly believe that god doesn't exist. 

                             Theoretical Studies of Atheism    

In the book, A Theory of Religion (1987), co-authored by Rodney Stark and William Bainbridge, the authors present their approach to religion, which became known as Stark-Bainbridge's theory of religion. Their theoretical approach to religion is based on the rational choice theory and social exchange theory. According to them, almost all human interactions can be considered a form of exchange. Religion serves to satisfy desires, that is, to provide rewards, and that can be, both for concrete things and for abstract needs (the meaning of life, the salvation of the soul). The investment in the realization of the award will be proportional to the size of the award. If people cannot easily achieve their goals, then they accept compensators, hope, and the promise that they will achieve those goals in the future, and that is exactly what religion gives them. Atheism spreads when religious compensators lose their appeal, especially in cases when churches and priests exploit the system for their own benefit.

In the book, The Sacred Canopy: Elements of a Sociological Theory of Religion (1967) Peter Berger studies secularity in modern society. Berger emphasizes the influence of Protestantism on the spreading of secular ideas because Protestantism encouraged a rationalist approach to religion and rejected mysticism and magic. In addition, the spread of secularism and non-religiousness is influenced by the "religious market", which is especially pronounced in the United States.

Books and articles:

Bainbridge, W. S. „Atheism“, in Interdisciplinary Journal of Research on Religion (2005);

Bruce, S. God is Dead: secularization in the West (2002);

Berger, P. The Sacred Canopy: Elements of a Sociological Theory of Religion (1967);

Campbell, Colin. Toward a Sociology of Irreligion (1971);

Cipriani, Roberto and Franco Garreli (ed.). Sociology of Atheism (2016);

Davenport, Thomas H. Virtuous Pagans: Unreligious People in America (1991);

Davie, Grace. Religion in Britain since 1945: Believing without Belonging (1994);

Dawkins, Richard. The God Delusion (2006);

Demerath, N. Jay, and Victor Thiessen. ''On Spitting Against the Wind: Organizational Precariousness and American Irreligion'', in American Journal of Sociology (1966);

Flynn, Tom, ed. The New Encyclopedia of Unbelief (2007); 

Harris, Sam. The End of Faith: Religion, Terror, and the Future (2005); 

Hitchens, Christopher. God is not Great. How Religion Poisons Everything (2007);

Hunsberger, Bruce E., and Bob Altemeyer. Atheists: A Groundbreaking Study of America's Nonbelievers (2006);

Inglehart, Ronald. Sacred and Secular: Religion and Politics Worldwide (2004);

M. Martin (Ed.) The Cambridge Companion to Atheism  (2007);

Russell, Bertrand. Why I am not a Christian, and other essays on religion and related subjects (1957); 

Stark. The Future of Religion: Secularization, Revival, and Cult Formation (1985);

Wilson, Bryan. Religion in Secular Society (1966).