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In addition to their roles as belief systems, religions are also socio-cultural systems which have often historically been tightly linked with governing authority, defining the , as well as leaders claiming to have been chosen or to represent their respective God(s). In spite of a trend towards in the 20^th century, whereby there is a strict separation between the religious organizations and the state, the turn of the 21^st century has seen some notable cases of growing religious influence in politics. Examples of this trend include hardline , the resurgence of Christian influence in former-Eastern Bloc states, , as well as Hindu nationalism in India. While in some regions, particularly Western Europe and East Asia, religious adherence is currently in sharp decline, the number of religious adherents as a share of world population is in fact predicted to increase over the coming century. These increases in religious adherents are largely driven by high fertility rates in regions such as Sub-Saharan Africa, where religious observance is stronger, and low fertility in more secular regions.

The birth of world religions

Archaeological evidence suggests that humans have showed proclivities towards belief in the supernatural long before the invention of organized religion, however the roots of today's major religions lie in early antiquity. Hinduism is believed to have emerged on the Indian subcontinent between the 15^th and 5^th centuries BCE, making it the oldest of the current world religions. The story of the emergence of Hinduism is somewhat opaque, as rather than having clear founding figures like most of the other world religions, it rather developed over a longer period, with its core religious texts, the Vedas, being developed over one thousand years by the priestly classes in the ancient language of Sanskrit. Buddhism was founded alongside Hinduism on the Indian subcontinent, with its founder Siddhartha Gautama, commonly known as the Buddha, living during the 5^th century BCE. While Buddhism experienced a period where it was dominant in South Asia, it later spread to East and Southeast Asia where it is at its strongest today.

The second eldest of the world religions is , which has a similarly long history, originating in the Levant sometime between the 9^th and 5^th centuries BCE. With Judaism's emergence we find the origin of the Abrahamic religions, the group of religions comprising of Christianity, Islam, and Judaism. While these three religions all believe in the same God, originating with the Jewish prophet Abraham, their religious practices, beliefs, and traditions all vary widely. and emerged long after Judaism, with Christians beginning to appear in the 1^st century, after the death of their prophet Jesus Christ, while Islam began to gain converts with their prophet Muhammad's conquests of the Arabian peninsula in the 7^th century. While Christians and Jews sometimes refer to their god by the name Yahweh, or its Latinized version Jehovah, Muslims do not have an individual name for God, referring instead to 'Allah', which is the Arabic word meaning 'god'.

Converts, conquests, and caliphates

As human civilization developed, cultural and commercial were forged. With these interactions came the spread of religion, which had previously been more localized. The oldest world religions managed to spread with the early development of civilization around river basins and coastlines, such as Hinduism's spread throughout the Indus Valley, the location of the largest of the ancient civilizations. While geography shaped the routes over which the world religions spread, the speed of their cultural diffusion across the globe was shaped by the social processes which spurred their adoption. Hierarchical diffusion, or the conversion of a monarch, emperor, or other political leaders, worked by making a specific religion the official state religion. For instance, Christianity became the state religion of the Roman empire with the conversion of Emperor Constantine in 312 CE, while Buddhism originally spread in India due to being adopted by the emperor Ashoka the Great in the 3^rd century BCE.

Religions have also diffused through expansion, with religious conquests being common throughout history. The early Muslim conquests spread Islam from the area of modern day Saudi Arabia, where the prophet Muhammad had originally lived, to all of the Middle East, North Africa, Central Asia, the Caucuses, and to Iberia (modern day Spain and Portugal). These caliphates, Islamic empires, which reigned in the centuries after Muhammad's death, have led to Islam being the predominant religion in most of these regions to this day. In medieval Europe, Christianity was sometimes mixed with Pagan beliefs to ease the transition to Christendom, however there was lower tolerance for other major religions. Conflict between Muslims and Christians in the Levant and Iberia in the Middle Ages also led to these religions becoming increasingly militant in the centuries that followed, and this aggressive strain continued to influence the spread of Christianity to the Americas, Asia, and (to a lesser extent) Africa in the early period of European colonialism. In some cases where the original religions of people were not suppressed following the uptake of Abrahamic religions, there was a fusion between the old and new, such as the combination of Buddhism and or the combination of Christianity and indigenous religious practices in parts of Mexico.

Beliefs and practices

The most common belief among world religions is a belief in a god or deities, supernatural figures which are often worshipped by believers. The Abrahamic religions all believe in one God, the god of Abraham, who first appears in the Torah, the Jewish religious scripture. Hinduism on the other hand is famous for having a host of different gods, with some accounts suggesting there are as many as 33 million. While many of these gods are related to specific places or practices, the three most prominent gods in Hinduism, Brahma, Vishnu, and Shiva, play important roles in the Hindu story of the creation of the Universe. Buddhism does not have any deity, making it quite different from the other world religions, with Buddhists instead believing in a philosophy of life which aims to achieve harmony with the world. Hinduism and Buddhism both believe in , the rebirth of the soul in a new form in the world after death. The Abrahamic religions, by contrast, believe that humans ascend to after death, eternal transcendent states which either reward a life lived well (heaven) or punish the sins committed while living (hell). Aside from beliefs about supernatural matters, world religions often proscribe behavior with rules about proper conduct, morality, and lifestyles.

Religious practice is often based around the attendance of regular sermons at places of worship, such as , , or synagogues for Jews. Hindus and Buddhists, while also having sacred temples and places of worships, tend to also worship in the home or in nature. Alongside regular worship, religions hold ceremonies based on times of the year (such as Winter, harvest or ), life events (birth, marriage, death), and specific religious milestones, such as commemorating Jesus Christ's death and resurrection or honoring Muhammad's first revelation. These ceremonies often involve , music, food, or meditation, among countless other religious rituals. A particularly notable practice are religious pilgrimages, whereby believers travel to a particular holy site in order to worship. The in Hinduism is the largest pilgrimage in the world, with up to 120 million people making the trip to bathe in the waters of rivers in India every 12 years, while the yearly pilgrimage to Mecca, Saudi Arabia, known as the Hajj, is one of the central pillars of Islam, and it is expected of Muslims that they must make the pilgrimage at least once in their lifetimes.

World religions today

Currently, the world's largest religion is Christianity, followed by Islam, Hinduism, and then Buddhism. Judaism, in spite of its significance as the foundational Abrahamic religion and the worldwide recognition of its beliefs, is followed by only 13 million people. The complex and tumultuous history of the Jews means that they are generally considered an ethnic group, as well as a religious group - the worldwide Jewish population was greatly reduced by the genocide perpetuated by Nazi Germany during the Second World War, known as or Shoah, with estimating that the population could be as high as 32 million today had the genocide not occurred. In the coming decades, , increasing its followers across the globe by around 70 percent. In spite of this phenomenal growth, . The only world religion forecasted to experience a decline in adherents up until 2050 is , due to demographic developments such as aging populations and low fertility rates in countries such as China, Japan, and South Korea.

The growth and decline of world religions is now tightly related to global demographics rather than conversion or conquest, as it was in the past. For instance, the increase in Christianity over the past century has been , which have much higher fertility rates than wealthier countries. In spite of its , this is why Christianity is still projected to grow as a share of world population up until 2060. While it appears that the current world religions will go from strength to strength in the coming century, there is also likely to be a proliferation of numerous new or smaller religions. Among these smaller world religions are , an Abrahamic religion which stresses the unity of all faith groups, , a monotheistic Indian religion, and numerous traditional folk religions such as those found in China or African countries. In many countries there has been an increase in interest in , which while religious in nature, does not usually come in the same form as organized religions.