Humanism is the belief that human life does have meaning and purpose, but that human beings themselves are the source and standard of positive human values. (This definition does not apply to the movement called Renaissance humanism.) Secularism is the belief that public, social, and governmental values should be devoid of religious influence.
The Enlightenment, an intellectual and cultural movement emerging toward the end of the eighteenth century, rejected the Bible and any other revelation and insisted on unguided human reason as the sole authority in all matters of knowledge. Enlightenment thinkers tended at first to hold to Deism, the belief that a Creator God made the universe but has no further involvement in it, but its method led quickly to atheism. The philosophers David Hume and Immanuel Kant criticized the standard arguments for God's existence. If we cannot know God's existence through either revelation or reason, the only basis left is religious or spiritual feeling or blind faith. Western thought since the Enlightenment has therefore tended in three directions. Some accept the Enlightenment critiques and reject belief in God (atheism). Some accept those critiques and base belief in God on feeling (pietism) or faith (fideism). Some reject those critiques and base belief in God on revelation or reason or both.
Atheism flowered in nineteenth-century German philosophy through such thinkers as Ludwig Feuerbach, who introduced the notion that God was an imagined father figure, and Friedrich Nietzsche, who declared famously that God was dead-meaning that humanity had supposedly understood that God did not exist after all. A key to the development of a thoroughly atheistic worldview, however, was the scientific theory of biological evolution by natural selection in the British scientist Charles Darwin's book The Origin of Species
(1859). Although not all evolutionists are atheists, atheism is intellectually untenable without some version of evolutionism. Since Darwin, theorists in the behavioral and social sciences have sought to apply the naturalistic assumptions of evolutionism to humanity. A notable example is the work of Sigmund Freud, who developed a theory of human psychology that was overtly atheistic. According to Freud, belief in God is a projection of the ideal father figure and thus a form of wish-fulfillment (compare Feuerbach). Most recently, some scientists have attempted to explain the mind, moral values, and even religious belief in purely biological terms.
Atheism became the worldview basis for powerful totalitarian ideologies in the twentieth century through the philosophy of the nineteenth-century atheist Karl Marx, whose Communist Manifesto
(1848) inspired both the Bolshevik (Leninist) Revolution in Russia, leading to the formation of the Soviet Union, and the Communist (Maoist) Revolution in China.