Short answers, pulled from the story.
Irreligion is the absence, rejection, or indifference toward religious beliefs or practices. Scholars disagree on definition: some restrict it to active rejection of religion, while others use it broadly to cover any lack of religious affiliation. The Encyclopedia of Religion and Society defines it as active rejection, distinguishing it from mere secularity.
As of the 2020 population estimate, irreligious individuals constitute approximately 24.2 percent of the global population, totalling around 1,905,360,000 people. In 2010, Pew Research Center counted more than 1.1 billion religiously unaffiliated worldwide.
China hosts the largest count, with more than 1.27 billion irreligious individuals as of 2020. The next largest counts are in the United States (more than 100 million), Japan (more than 70 million), Vietnam (more than 60 million), and Germany (more than 30 million). These five countries, along with Russia, Brazil, France, the United Kingdom, and South Korea, together account for 88.5 percent of the global irreligious population.
Atheism is the lack of belief that any deities exist; positive atheism is the specific claim that no deities exist. Agnosticism holds that the existence of God is unknown or unknowable. Irreligion is the broader umbrella term covering both, as well as apatheism, ignosticism, religious skepticism, and many other positions involving absence of or opposition to religion.
Pew Research Center projects that the irreligious share of the world population will decline from 16.4 percent to 13.2 percent by 2050, because highly religious countries have faster population growth. Between 2007 and 2019, however, 43 out of 49 countries studied became less religious, suggesting that declining religiosity is spreading even as the unaffiliated population-share shrinks.
The word irreligion first appeared in French as irréligion in 1527. The English form was first attested in 1598. Dutch borrowed it as irreligie during the 17th century, though it is not certain whether that borrowing came from French or from English.