Practically, a state without a state religion is called a secular state. The term state church is associated with Christianity, and is sometimes used to denote a specific national branch of Christianity. Closely related to state churches are what sociologists call ecclesiae, though the two are slightly different. State religions are examples of the official or government-sanctioned establishment of religion, as distinct from theocracy. It is also possible for a national church to become established without being under state control.The first national church was the Armenian Orthodox Church which was established in 301 A.D.
In these cases, state religions are widely seen as efforts by the state to prevent alternate sources of authority.
A "state church" is created by a monarch, as in the cases of the Anglican Church, created by Henry VIII or the Church of Sweden, created by Gustav Vasa. An example of "state religion" is Argentina's acceptance of Catholicism as its religion.
But the population's allegiance towards the state religion is often strong enough to prevent them from joining competing religious groups.
Many of the early Sumerian rulers were priests of their patron city god. Some of the earliest semi-mythological kings may have passed into the pantheon, like Dumuzid, and some later kings came to be viewed as divine soon after their reigns, like Sargon the Great of Akkad. One of the first rulers to be proclaimed a god during his actual reign was Gudea of Lagash, followed by some later kings of Ur, such as Shulgi. Often, the state religion was integral to the power base of the reigning government, such as in Egypt, where Pharaohs were often thought of as embodiments of the god Horus.
However, it persisted as the state religion of the independent state of Hyrcania until the 15th century.
Despite enjoying considerable popular support, Christianity was still not the official state religion in Rome, although it was in some neighboring states such as Armenia and Aksum.
Confucianism would continue on as the state religion until the Sui Dynasty (581-618), when it was replaced by Buddhism. Neo-confucianism returned as the de facto state religion sometime in the 10th century. Note however, there is a debate over whether Confucianism (including Neo-confucianism) is a religion or purely a philosophical system.
Source: Wikipedia > State Religion
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