Atheism is rejecting the belief in a god or gods. It is the opposite of theism, which is the belief that at least one god exists. A person who rejects belief in gods is called an atheist. Theism is the belief in one or more gods. Adding an a, meaning "without", before the word theism results in atheism, or literally, "without theism".
Atheism is not the same as agnosticism: agnostics say that there is no way to know whether gods exist or not. Being an agnostic does not have to mean a person rejects or believes in god. Some agnostics are theists, believing in god. The theologian Kierkegaard is an example. Other agnostics are atheists. Gnosticism refers to a claim of knowledge. A gnostichas sufficient knowledge to make a claim. Adding an a, meaning "without", before the word gnostic results in agnostic, or literally, "without knowledge".
While theism refers to belief in one or more gods, gnosticism refers to knowledge. In practice, most people simply identify as a theist, atheist, or agnostic.
Anaxagoras was the first known atheist. He was an Ionian Greek, born in Clazomenae in what is now Asia Minor. He travelled to other Greek cities, and his ideas were well known in Athens. Socrates mentioned that his works could be bought in Athens for a drachma. Eventually he was prosecuted and condemned for impiety, and banished from Athens.
Anaxagoras' beliefs were interesting. He thought the Sun was not a god, and was not animated (alive). The Sun was "a red-hot mass many times larger than the Peloponnese". The Moon was a solid body with geographical features, and made of the same substance as the Earth. The world was a globe (spherical).
Atheists often give reasons why they do not believe in a god or gods. Three of the reasons that they often give are the problem of evil, the argument from inconsistent revelations, and the argument from nonbelief. Not all atheists think these reasons provide complete proof that gods cannot exist, but these are the reasons given to support rejecting belief that gods exist.
Some atheists do not believe in any god because they feel that there is no evidence for any god nor gods and goddesses, so believing any type of theism means believing unproved assumptions. These atheists think a simpler explanation for everything is methodological naturalism which means that only natural things exist. Occam's razor shows simple explanations without many unproved guesses are more likely to be true.
In everyday life, many people define natural phenomena without the need of a god or gods. They do not deny the existence of one or more gods, they simply say that this existence is not necessary. Gods do not provide a purpose to life, nor influence it, according to this view. Many scientists practice what they call methodological naturalism. They silently adopt philosophical naturalism and use the scientific method. Their belief in a god does not affect their results.
Practical atheism can take different forms:
Absence of religious motivation—belief in gods does not motivate moral action, religious action, or any other form of action;
Active exclusion of the problem of gods and religion from intellectual pursuit and practical action;
Indifference—the absence of any interest in the problems of gods and religion; or
Unawareness of the concept of a deity.