The term Sohei means samurai (warriors) with the appearance of priests who were active from the Medieval Period until the early modern ages.
Hoshi-musha or armed priests were called Soshu or Akuso in the era when they were active but since the Edo period, they have been called "Sohei." Incidentally, the Chinese character of "悪(aku)" used for the term of 悪僧 (Akuso), like 悪 (aku) used for 悪党 (akuto) means "strong." They basically belonged to temples as armed groups. Their figures were depicted in picture scrolls and their characteristics were cloth covering heads, Takageta (tall wooden clogs) and Naginata (halberd). It is highly possible that they didn't shave their heads. In the meantime, armed groups belonging to shrines were called Jinin. As there were Buddhist priests' groups outside of Japan that organized priests into armed groups, such as Suzan Shorin-ji Temple (Mountain Sung Shaolin Temple), the term Sohei sometimes refers to such armed religious groups in broad terms. When used in this sense, the Military Order in Europe is also included in the category of Sohei.
During the era when Sohei or Jinin were active, temples and shrines were rich due to holding vast Jiryo (landownership of Buddhist Temples) or Shrine Estates amidst the unstable society and they faced risks of being assailed by various social groups, including groups of robbers. For this reason, the need to have military power for defending temples and shrines from such attacks arose. In this historic context, 'arming of temples and shrines,' although it seemingly sounds contradictory, was in fact promoted.
Historically, Daishu (Doshu), those who belonged to big temples in Kyoto and Nara City and were engaged in odd jobs, and armed themselves for self-defense and became Sohei. During the late Heian period, Sohei became powerful armed groups with their stronghold including at Kofuku-ji Temple, Enryaku-ji Temple, Onjo-ji Temple and Todai-ji Temple and were repeatedly involved in power struggles among temples or in acts of menace to the imperial court and Sekke (line of regents and advisers). In Mochihito-o War, Sohei also fought against the Taira family. Such a fight against the Taira family is depicted in "Heike Monogatari" (The Tale of the Taira family). Sohei belonging to Kofuku-ji Temple (Nanto, southern capital) and Enryaku-ji Temple (Hokurei) were specifically called Shuto (Nara-hoshi) and Yama-hoshi respectively. Emperor Shirakawa cited 'water of the Kamo River (flow of Kamo-gawa River), cast of sugoroku and Yama-hoshi (Mt. Hiei)' as being beyond his control and from this anecdote, it can be guessed that the high-handedness of the Sohei was a worry for the imperial court.
Even in regions far from the center of Japan, some dominant temples and shrines exerted a big influence on the power balance in those days with holding military power or by joining forces with the local militia. It is well- known that during Jisho-Juei War, both sides made political deals with Kumano Gongen Deity that presided over Kumano Suigun Navy.
In the Muromachi period, Yoshinori ASHIKAGA, who was formerly known by Gien and had once been a temple head priest of the Tendai sect, was familiar with the military power and high-handedness of Sohei to conduct massive military operations in order to crack down on Enryaku-ji Temple (Nobunaga conducted similar operations later).
After that, such trend of those dominant temples and shrines in various regions to hold military power continued. The following are examples of powerful Sohei groups during the Sengoku Period (Period of Warring States).
Negoro-ji Temple; it was well-known for its mighty harquebus corps including Kazunaga TSUDA, the founder of Tsuda school of gunnery. It was subdued by Hideyoshi TOYOTOMI by his expedition to Kishu and lost military power.
Ishiyama Hongan-ji Temple; it boasted overwhelming influential power as well as military power through Ikko Ikki (an uprising of Ikko sect followers) which was fought by, instead of its own Sohei group, believers called up from every region of the country (many of them were indigenous persons and ordinary people), but eventually it suffered devastating defeat in the Ishiyama War and lost its military power.
Usa-jingu Shrine; it was subdued by Yoshishige OTOMO and lost military power.
Since all temples and shrines mentioned above had either lost their military power or had been divided, their religious powers had been separated from the military powers by the Edo period.
Rinno-ji Temple on Mt. Nikko; it held many Sohei and was one of the leading armed groups in Shimotsuke Province. Because it joined forces with the Gohojo clan, it was oppressed by Hideyoshi TOYOTOMI to be forfeited its Jiryo, then declined. Later, it recovered its clout under the protection of Ieyasu TOKUGAWA who had been transferred to Kanto.
Heisen-ji Temple Hakusan-jinja Shrine; as the branch temple of Enryaku-ji Temple, it held eight thousand Sohei and extended its power to Echizen Province. In 1574, it was burnt down in the battle against Ikko Ikki. It was declined thereafter. However, by allying itself with Nobunaga ODA and Hideyoshi TOYOTOMI from early on, it recovered its Jiryo.