Atheism, from the Greek a (‘without’) and the os (‘deity’), commonly and loosely refers to the theoretical or practical denial of the existence of a deity. The concrete meaning of atheism has varied considerably in history: even the earliest Christians were labeled ‘atheists’ because they denied the existence of the Roman deities. In Western culture, where monotheism has been the dominant mode of religious belief, atheism has generally referred to the denial of the existence of a transcendent, perfect, personal creator of the universe. To be an atheist need not mean that one is nonreligious, for there are ‘high’ religions, such as Buddhism and Taoism, that do not postulate the existence of a supernatural being. Monotheism has been so basic to and compounded with Western moral and philosophical beliefs as well as political institutions that until recently atheism has been widely believed to be both immoral and dangerous to society. Plato not only viewed atheism as irrational but argued that certain atheists deserved the death penalty.
When Christianity finally became the dominant religion in the West, atheism and heresy were thought to be worthy of exile or death because, as Thomas Aquinas argued, it was a much more serious matter to corrupt the soul than to damage the body. Atheism was also dangerous to the political authority of Western monarchies that claimed to rest upon divine right. Even during the Enlightenment when the divine right of kings was challenged and religious toleration defended, John Locke, a staunch advocate of toleration, denied free speech to atheists on the grounds that they undermined and destroyed religion. The believability of atheism seems directly proportionate to the growth of the sciences and the emergence of humanism since the Renaissance. In the 19 th century the biological sciences seemed to make theological explanations of the origins of the universe and of the emergence of humankind unnecessary.
Particularly important were the writings of David Hume and Immanuel Kant, which established that attempts to prove the existence of God from the world order were invalid. In the mid-19 th century, explicitly atheistic and humanistic systems of philosophy appeared. Ludwig Feuerbach, Karl Marx, Arthur Schopenhauer, and Friedrich Nietzsche were not only atheists but also militant critics of religion generally and of Christianity particularly. Modern philosophical atheism is based on both theoretical and practical reasons. Theoretically, atheists argue either that there are no good arguments for believing in the existence of a personal deity, whether this deity be conceived of anthropomorphically or metaphysically, or that the statement God exists is incoherent or meaningless.
The last type of logical criticism of theism is characteristic of logical positivism and analytic and linguistic philosophy. Practically, some atheists have argued, as did Nietzsche, that belief in a supernatural and supreme being requires a devaluation of this life; or, as Freud did, that the belief is an expression of infantile helplessness.