Democritus (c. 460 – c. 370 BCE) was an Ancient Greek philosopher born in Abdera, Thrace, Greece. A pupil of Leucippus, he was an influential pre-Socratic philosopher who formulated an atomic theory for the universe. Being a materialist philosopher criticizing contemporary religious beliefs, he is said to be the first thinker to introduce the concept of ‘atom’, a rigorous scientific concept that could not be actually discovered before twentieth century.

Galileo Galilei (15 February 1564 – 8 January 1642), often known mononymously as Galileo, was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His most famous achievements include improvements to the telescope and consequent astronomical observations, discovering fundamental laws of mechanical motion and support for Copernicanism. Galileo has been called the "father of modern observational astronomy", the "father of modern physics", the "father of science", and "the Father of Modern Science".Galileo's championing of heliocentrism was controversial within his lifetime. He was tried by the Inquisition, found "vehemently suspect of heresy", forced to recant, and spent the rest of his life under house arrest.

François-Marie Arouet (21 November 1694 – 30 May 1778), known by his nom de plume Voltaire, was a French Enlightenment writer, historian and philosopher famous for his wit, his attacks on the established Catholic Church, and his advocacy of freedom of religion, freedom of expression, and separation of church and state. He is considered to be one of the founders of modern secularism.  Voltaire was a versatile writer, producing works in almost every literary form, including plays, poems, novels, essays, and historical and scientific works. He wrote more than 20,000 letters and more than 2,000 books and pamphlets. He was an outspoken advocate, despite the risk this placed him in under the strict censorship laws of the time. As a satirical polemicist, he frequently made use of his works to criticize intolerance, religious dogma, and the French institutions of his day.

Claude Adrien Helvétius (26 January or 26 February 1715 (sources differ) – 26 December 1771) was a rationalist French philosopher and littérateur. Helvétius' philosophy held that man’s faculties are interest-driven but rational, and hence there’s no place for the supernatural or the religious. It held, for example, that all man's faculties may be reduced to physical sensation, even memory, comparison, judgment. So self-interest, founded on the love of pleasure and the fear of pain, is the sole spring of judgment, action, and affection. Human beings are motivated solely by the pursuit of pleasure and the avoidance of pain. And also, we have no freedom of choice between good and evil. There is no such thing as absolute right – ideas of justice and injustice change according to customs. This view of man was largely Hobbesian – man is a system deterministically controllable by a suitable combination of reward and punishment, and the ends of government are to ensure the maximisation of pleasure.

Charles Robert Darwin, FRS (12 February 1809 – 19 April 1882) was an English naturalist and geologist, best known for his contributions to evolutionary theory. He established that all species of life have descended over time from common ancestors, and in a joint publication with Alfred Russel Wallace introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding. Darwin has been described as one of the most influential figures in human history. Though not at all bothered to talk about religion, he is still regarded as the one who offered the biggest blow to religion. By asserting that man appeared according to natural processes without any divine intervention, he helped demolish religious creationism once and for all.

Friedrich Engels (28 November 1820 – 5 August 1895) was a German social scientist, author, political theorist, philosopher, and father of Marxist  theory,  alongside Karl Marx. In 1845 he published The Condition of the Working Class in England, based on personal observations and research. In 1848 he co-authored The Communist Manifesto with Karl Marx, and later he supported Marx financially to do research and write Das Kapital. After Marx's death, Engels edited the second and third volumes. Additionally, Engels organized Marx's notes on the "Theories of Surplus Value" and this was later published as the "fourth volume" of Capital. He has also made important contributions to family economics. He also developed a Marxist understanding of the content and history of science and religion.

Bertrand Arthur William Russell, 3rd Earl Russell, OM, FRS (18 May 1872 – 2 February 1970) was a British philosopher, logician, mathematician, historian, and social critic. At various points in his life he considered himself a liberal, a socialist, and a pacifist, but he also admitted that he had never been any of these in any profound sense. He was born in Monmouthshire, into one of the most prominent aristocratic families in Britain. Russell led the British "revolt against idealism" in the early 20th century. He is considered one of the founders of analytic philosophy along with his predecessor Gottlob Frege and his protégé Ludwig Wittgenstein. He is widely held to be one of the 20th century's premier logicians. He co-authored, with A. N. Whitehead, Principia Mathematica, an attempt to ground mathematics on logic. His philosophical essay "On Denoting" has been considered a "paradigm of philosophy". His work has had a considerable influence on logic, mathematics, set theory, linguistics, artificial intelligence, computer science (see type theory and type system), and philosophy, especially philosophy of language, epistemology, and metaphysics. Russell was a also a prominent anti-war activist; he championed anti-imperialism and went to prison for his pacifism during World War I. Later, he campaigned against Adolf Hitler, then criticized Stalinist totalitarianism, attacked the involvement of the United States of America in the Vietnam War, and was an outspoken proponent of nuclear disarmament. In 1950 Russell was awarded the Nobel Prize in Literature "in recognition of his varied and significant writings in which he champions humanitarian ideals and freedom of thought.” His eloquent and humorous critiques against religious irrationality, obscurantism and authoritarianism make him one of the greatest rationalist thinkers of all time.

Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He spent most of his career as a professor of astronomy at Cornell University where he directed the Laboratory for Planetary Studies. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books. He advocated scientific skeptical inquiry and the scientific method, pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence (SETI). Sagan is known for his popular science books and for the award-winning 1980 television series Cosmos: A Personal Voyage, which he narrated and co-wrote. The book Cosmos was published to accompany the series. Sagan wrote the novel Contact, the basis for a 1997 film of the same name. In his greatly enjoyable popular work, he always argued in favor of rational and scientific worldview and eloquently opposed superstition, pseudoscience, charlatanry and all attempts at interpreting scientific facts and theories in a manipulative and obscurantist way. His popular works are regarded as part and parcel of contemporary rationalist movement.