The concept of state religions was known as long ago as the empires of Egypt and Sumer, when every city state or people had its own god or gods. 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.

http://en.wikipedia.org/wiki/State_religion#Ancient_state_re...

The era of Christian hegemony began with the conversion of Constantine I. In 391, Christianity became the state religion of Rome under Theodosius I, to the exclusion of all other cults. Beginning with the patristic writers of the 4th century, the diverse traditional religions practiced throughout the Empire were condemned collectively as "pagan",[1] and were gradually transformed, absorbed or suppressed.

http://en.wikipedia.org/wiki/Religion_in_ancient_Rome