Denying the existence of God or ignoring His existence as irrelevant, or not believing in someone else's God.
Plato used the word in his writings; it appears in the epistles of St. Paul; and early Christian Church fathers used it to describe pagans. The Romans used it to describe Christians. From the origin of Christianity until the 15th century, explicit atheism was practically unknown. The Renaissance and its emphasis on humanism fostered a return to explicit atheism. Political philosophers of the 18th century tended to deny the divine right of kings. However, some reserved the divine right of individual free choice. Nietzsche postulated the death of God and the arrival of superman. Humanism and modern scientific theory are seen by some religious bodies as atheism. They include as atheistic some eastern religions such as Buddhism, Jainism and Confucianism which accept first principles other than a personal god. The Christian Church recognizes Marxism as the modern standard for political atheism on the foundations of moralism. They face the dilemma of distinguishing right from wrong in the absence of belief in a god.
Modern experimenters in social, political and physical science try to remove the impediments of religious considerations to proceed in their work under their own and secular ethical constraints.
Atheism has contributed to the development of modern political systems by opposing either social needs or human values to the former god-given rights of rulers and political leaders.
It has contributed to the active pursuit of scientific knowledge by placing a higher value on intellectual enlightenment than on religious discipline.
Atheism attacks the foundations of any ethical system based on the acceptance of supernatural powers. Personal choice or humanistic consensus, when substituted for religious principles, weaken ethical discipline. Social destruction can result from undisciplined choices of action, such as nuclear warfare.
Every human being owes their allegiance to some final belief or god and this is demonstrated by people who, in advanced maturity, return to religious practices and ethics.