Atheism (or non-theism) is the belief that gods do not exist, or a complete rejection of Theism or any belief in a personal god or gods (the latter also known as antitheism). It can cover a range of both religious and nonreligious attitudes. Many atheists tend toward secular philosophies such as Humanism and Naturalism.

The term "atheism" (from the Greek "godless") originated as an insult applied to any person or belief in conflict with established religion, the first English usage dating back to the 16th Century. In common use, it merely indicates a disbelief in God, rather than an active denial of the existence of any gods. With the spread of freethought, scientific skepticism and criticism of religion, the term began to gather a more specific meaning and was first used to describe a self-avowed belief in late 18th Century Europe, and is now increasingly used as a self-description by atheists.

Several religions, including Confucianism, Taoism, Jainism and some varieties of Buddhism, either do not include belief in a personal god as a tenet of the religion, or actively teach non-theism.

In Ancient Greece, the 5th Century B.C. philosopher Diagoras is often credited as the "first atheist" and strongly criticized all religion and mysticism. Atomists such as Democritus attempted to explain the world in a purely materialistic way, without reference to the spiritual or mystical. Epicurus disputed many religious doctrines, including the existence of an afterlife or a personal deity and, while he did not rule out the existence of gods, he believed that if they did exist they were unconcerned with humanity. Skeptics like Pyrrho and Sextus Empiricus held that one should suspend judgment about virtually all beliefs.

During the Middle Ages, Scholasticism and orthodoxy in religious thought was at its height, and Atheism was a very uncommon, even dangerous, doctrine, although William of Ockham went so far as to assert that the divine essence could not be intuitively or rationally apprehended by human intellect. By the time of the Renaissance (15th - 16th Centuries), more skeptical inquiry was beginning and Niccolò Machiavelli, Leonardo da Vinci, Bonaventure des Périers and François Rabelais all criticized religion and the Church during this time.

In 17th and 18th Century Europe, Deism increased in popularity and criticism of Christianity became increasingly frequent, but it was only towards the end of the 18th Century that Atheism began to be openly espoused by individuals such as Jean Meslier and Baron d'Holbach, and the Empiricist David Hume began to undermine the metaphysical basis of natural theology.

By the mid-19th Century, many prominent German philosophers (including Ludwig Feuerbach, Arthur Schopenhauer, Karl Marx and Friedrich Nietzsche) were denying the existence of deities and were strongly critical of religion.

In the 20th Century, atheistic thought found recognition in a wide variety of other broader philosophies, such as Existentialism, Objectivism, Humanism, Nihilism, Logical Positivism and Marxism, as well as the Analytic Philosophy, Structuralism, Naturalism and Nominalism movements they gave rise to. Bertrand Russell emphatically rejected belief in God, and Ludwig Wittgenstein and A. J. Ayer, in their different ways, asserted the unverifiability and meaninglessness of religious statements.

New Atheism is a social and political movement that began in the early 2000s in favour of atheism and secularism. It has been largely promoted by a handful of popular radical atheist writers, including the so-called "Four Horsemen of the Non-Apocalypse": Richard Dawkins (1941 - ), Christopher Hitchens (1949 - 2011), Sam Harris (1967 - ) and Daniel Dennett (1942 - ). The movement advocates the view that "religion should not simply be tolerated but should be countered, criticized and exposed by rational argument wherever its influence arises".

Implicit Atheism is the absence of belief in one or more gods, without a conscious rejection of it. This may apply to someone who has never thought about belief in gods, or never been exposed to theistic ideas, or, some would argue, also to newborn children. Explicit Atheism, on the other hand, is where someone makes a positive assertion, either weak or strong, regarding their lack of belief in gods.

Another distinction is sometimes made between strong (or positive) atheism and weak (or negative) atheism. Strong atheism is a term generally used to describe atheists who accept as true the proposition "gods do not exist". Weak atheism refers to any type of non-theism which falls short of this standard, and which can therefore be considered to also include Agnosticism.

A third distinction can be made between practical (or pragmatic) atheism, and theoretical (or contemplative) atheism. In practical atheism (also known as apatheism), individuals live as if there are no gods and explain natural phenomena without resorting to the divine. This may be from an absence of religious motivation; an active exclusion of the problem of gods and religion from intellectual pursuit and practical action; indifference and lack of interest in the problems of gods and religion; or just ignorance or a lack of any idea about gods. Theoretical atheism, on the other hand, explicitly posits arguments against the existence of gods, and actively responds to the common theistic arguments (see the section on Philosophy of Religion).

Some atheists argue a lack of empirical evidence for the existence of deities and are are skeptical of all supernatural beings, while others argue for Atheism on philosophical, social or historical grounds.

Among the arguments for atheism are:

• Epistemological arguments:
  Various arguments claim that people cannot know God or determine the existence of God (arguably equivalent to Agnosticism). The rationalistic agnosticism of Kant only accepts knowledge deduced with human rationality, and holds that gods are not discernible as a matter of principle, and therefore cannot be known to exist. Skepticism asserts that certainty about anything is impossible, so one can never know the existence of God. Logical Positivism asserts the meaninglessness or unintelligibility of basic terms such as "God" and statements such as "God is all-powerful". Non-cognitivism holds that the statement "God exists" does not express a proposition and is therefore nonsensical or cognitively meaningless.
• Metaphysical arguments:
  Absolute metaphysical atheists subscribe to some form of Physicalism, which explicitly denies the existence of non-physical beings. Relative metaphysical atheists maintain an implicit denial of a particular concept of God based on the incongruity between their individual philosophies and attributes commonly applied to God, such as transcendence, personal aspect, unity, etc.
• Psychological, sociological and economical arguments:
  Some thinkers, including the anthropologist Ludwig Feuerbach and the psycologist Sigmund Freud, have argued that God and other religious beliefs are human inventions, created to fulfill various psychological and emotional wants or needs. Marxists like Karl Marx and Friedrich Engels and the Russian anarchist and revolutionary Mikhail Bakunin have argued that belief in God and religion are social functions, used by those in power to oppress and enslave the working classes.
• Logical and evidential arguments:
  Logical atheism holds that the various conceptions of gods, such as the personal god of Christianity, are ascribed logically inconsistent qualities(such as perfection, omniscience, omnipotence, omnibenevolence, transcendence, personhood, etc). Epicurus is credited with first expounding the problem of evil (the problem of reconciling the existence of evil or suffering in the world with the existence of a god - see the section in Philosophy of Religion), although a similar argument is also attributed to Siddhartha Gautama, the founder of Buddhism.
• Anthropocentric arguments:
  Axiological (or constructive) atheism favors humanity as the absolute source of ethics and values, and permits individuals to resolve moral problems without resorting to God. Marx, Nietzsche, Sartre and Freud all used this argument to some extent to convey messages of liberation, full-development, and unfettered happiness.