In regard to the multiplicity of religions, the following works provide a good reference:
John Ferguson. Gods Many and Lords Many: A Study in Primal
Religions. Guildford, Surrey: Lutterworth Educational, 1982.
Suan Imm Tan. Many Races, Many Religions. Singapore: Educational
Publications Bureau, 1971-72.
H. Byron Earhart. Religions of Japan: Many Traditions within One
Sacred Way. San Francisco: Harper & Row, 1984.
John M. Reid. Doomed Religions. A Series of Essays on Great
Religions of the World. New York: Phillips & Hunt, 1884.
Although no precise statistics are available, it is assumed that ca. three billion people acknowledge religion in our days. The numbers are misleading, though. For instance, only 2.4% of the population in England attends religious services; in Germany, the percentage is 9%; in some Moslem countries, service attendance is close to 100%. The "3-day Jews" (two days of Rosh Hashana and 1 day of Yom Kippur, also known as "revolving door" Jews, in for New Year and out after Atonement Day), the Christian Orthodox and Catholics of Christmas and Easter, and the Buddhists of funeral ceremonials belong to the vast majority that refers to religion as a cultural identifier. Many priests and higher order ecumenical workers recite their prayers as epic poetry.
Atheism. The "doctrine that God does not exist, that existence of God is a false belief" (cf. M. Eliade, Encyclopedia of Religion, vol. 1, pp 479-480). Literature on atheism continuously increases. A selection showing the many angles of atheism can serve as a guide:
The American Atheist (periodical). Austin TX: American Atheists.
Gordon Stein, Editor. An Anthology of Atheism and Rationalism.
Buffalo NY: Prometheus Books, 1980.
Michael Martin. Atheism: A Philosophical Analysis. Philadelphia:
Temple University Press, 1990.