Astrianism, known officially as the Astral Church, is a dualistic religion based upon the teachings of the human physicist and prophet Svyatoslav, also known as the Lantern. It exalts the divinity of stars and celestial bodies, and tells of their struggle against Erebus, the personification of the void, darkness, and evil. It also predicts the coming of the Last Dawn, a cataclysmic conflict between light and dark, after which the universe will be reborn free of evil with the aid of a messianic hero, known as the Morningstar.

Various cults dedicated to the worship of individual stars exist under the umbrella of Astrianism. The largest of these is the cult of Sol Invictus, the patron of mankind, as well as the personification of glory in battle and wisdom, although numerous regional cults exist for lesser-known stellar bodies. Despite this, the Church is a highly hierarchical organisation, led by a Supreme Patriarch. Beneath the Supreme Patriarch are bishops and priests, while Star Readers sit at the bottom of the hierarchy. Despite their low rank, Star Readers are incredibly important to the functioning of the Church, being the primary interpreters of the gods will as well as skilled stellar navigators. While the rest of the church is male-dominated, all Star Readers are women.

Astrianism's teachings and core tenets are enshrined in the Carmina, also known as the Lantern's Song, the faith's scripture. It is composed primarily of the Lantern's own writings - liturgical poems inspired by Sol Invictus himself - but also includes passages composed by his disciples, which describe Dromosia or Holy Law. The philosophy of the Carmina is primarily inspired by Astrology, but also draws from ancient Terran faiths such as Christianity, Islam, Gnosticism, Satanism, and Zoroastrianism. It asserts that the Lantern is but one of many human prophets to receive revelations throughout the history of the species - previous prophets include Adam, Abraham, Zoroaster, Jesus, and Muhammed.

According to the Carmina, Astrianism was founded in the early Third Century BE by Svyatoslav, who was a physicist and astronomer before receiving his revelation from Sol Invictus in 272 BE. He established a base of operations on Belgorod, an Imperial mining colony, where the Astral Church was officially established in 246 BE, with Svyatoslav's favourite apostle, Mechyslav Macarius, serving as the first Supreme Patriarch. As the early church gained followers, it became the subject of persecution by the Universal Empire, leading to the First Astrian War in 24 BE. As a result of their military defeat, many Astrians then chose to flee to the fringes of the galaxy, in an event known as the Exodus - the start of which marks the year 0 in the Astrian Calendar. These Astrian exiles made their faith a dominant force within the Wild Fields during the First and Second Centuries AE, and it continues to be widely practised throughout the region.

At present, the Astral Church is the single most powerful organization within Zirkovia. Most fleets rely on star readers to navigate the galaxy, and the Church is guaranteed a seat on the Central Military Rada - Zirkovia's main legislative body. Its headquarters, the Astral Temple, is one of the largest craft ever built by Zirkovian hands - at almost 10,000 km in diameter, it easily rivals most colony ships fielded by the Host. The Church also maintains its own private army, the Sovereign Sisters of Saint Macarius, otherwise known as the Macarian Guard, a militia of female astromancers responsible for the Astral Temple's security and the persecution of heretics and non-humans. The Guard, while small, is regarded as one of the best fighting forces in all of Zirkovia, perhaps even capable of taking on all the Fleets together, although this has never been tested.