Short answers, pulled from the story.
Atheism, in the broadest sense, is an absence of belief in the existence of deities. More narrowly, it is a rejection of the belief that any deities exist, and in its narrowest sense the specific position that there are no deities. It is contrasted with theism, the belief that at least one deity exists.
Positive, or strong, atheism is the explicit affirmation that gods do not exist. Negative, or weak, atheism includes all other forms of non-theism. Philosophers Antony Flew and Michael Martin drew this distinction, under which anyone who is not a theist is either a negative or a positive atheist.
Estimates of those who have an absence of theistic belief range from 500 million to 1.1 billion people. A 2010 survey published in Encyclopaedia Britannica found the non-religious made up about 9.6% of the world's population and atheists about 2.0%. Win-Gallup International recorded convinced atheists at 13% in 2012-11% in 2015, and 9% in 2017.
Arguments for atheism range from philosophical to scientific to social approaches. They include the lack of evidence, the problem of evil, the argument from inconsistent revelations, the rejection of concepts that cannot be falsified, and the argument from nonbelief. Atheists also contend that the burden of proof lies on the theist, since everyone is born without belief in deities.
Atheistic viewpoints trace back to classical antiquity and early Indian philosophy. In India, schools such as Samkhya, the early Mimamsa, and the materialistic Charvaka, which originated around the sixth century BCE, did not accept God. In the Western world, atheism declined after Christianity gained prominence, then resurged in the 16th century and the Age of Enlightenment.
The problem of evil holds that an omniscient, omnipotent, and omnibenevolent God cannot be reconciled with a world containing evil and suffering. Epicurus is credited with first expounding it, and David Hume cited him in his Dialogues concerning Natural Religion of 1779, asking whether God is willing to prevent evil but not able, or able but not willing.
State atheism emerged in the Soviet Union under Vladimir Lenin and Joseph Stalin and in Communist China under Mao Zedong, with measures such as outlawing religious instruction and forming the League of Militant Atheists. Atheists including Sam Harris and Richard Dawkins have argued that Stalin's atrocities were driven by dogmatic ideology rather than by atheism, and were not done in atheism's name.