The Ecclesiarchy, also known as the Adeptus Ministorum, is the official state church of the Imperium of Man. Their worship of the God-Emperor and their ability to declare anyone who doesn't align with their religion a heretic makes them the single most powerful organization in the empire. Any idea, text, art, or action that does not meet with the approval of the church is deemed heretical and can and has ended with entire worlds behind put to the sword and the flame. They have entire worlds dedicated to the worship of the God-Emperor, where their priests live like royalty in palaces disguised as temples. They drape themselves in the finest silks and jewels and preach the truth of the Cult Imperialis.
The key word there is cult. They openly call themselves a cult. That used to be a sign of utter insanity, but in this age, it is the only legally accepted normality. The craziest part is their most sacred text, the work that started worship of the Emperor as a god: the Lectitio Divinitatus was written over ten thousand years ago by the Traitor Primarch Lorgar, one of the architects of the apocalyptic war known as the Horus Hersey which set the Imperium on the road to ruin and led to Emperor rotting on the Golden Throne. So, in truth, their entire religion is a lie that has become the only truth allowed and is run by fanatics and zealots who will kill anyone who dares to speak common sense. Then again, that is pretty much every religion in human history.
The Adeptus Ministorum has its own standing military force known as the Adepta Sororitas or the Sisters of Battle. This force consists of multiple orders of power-armored women taken as children and raised by the church to believe that their lives are meaningless without complete faith in the God-Emperor and unquestioning loyalty to his church. On the battlefield, they are potent warriors, but their combat doctrine basically amounts to pray hard and charge. If you die, then the God-Emperor willed it. When it comes to insanity, fanatism, and zealotry, they excel in every way. But they are just the official military force. Unofficially, the church calls upon whoever it feels like. The priests are the true leaders of the Imperium and are arguably the main reasons it and humanity are dying on the altar of Chaos.
Terra itself is little more than a shrine world. The most divine of all the shrine worlds. The world where the God-Emperor sits on his Golden Throne. It is the heart of the Imperium, where billions of pilgrims come to glimpse the birth world of humanity and lay eyes on the Imperial Palace. They want to pray in one of the thousands of temples that cover its surface and proclaim their love for the God-Emperor. Any who do so will proclaim it is a glorious experience. Well, any who are ever heard from again.
The reality is Terra is a barren, contaminated husk of its former self. The oceans have been boiled away. The forests are now eradicated wastelands. And it is choked by hive cities and churches. Of the innumerable billions that call it home now, barely one percent can say they have lives worth living. The upper class is comprised of the official servants of the Emperor and His Imperium, including Imperial officials, Ecclesiarchy clergy, the aristocratic families of the Navis Nobilite, military officers, scribes, and bureaucrats. The teeming masses who comprise the Terran lower classes are far less privileged, and many of them are nothing more than serfs or thralls who toil in the Throneworld's countless manufactoria, providing the infrastructure that keeps the heart of the Imperium's byzantine bureaucracy functioning. In plain terms, it is Hell on Earth.
The beheading of the Adeptus Ministorum started when Ecclesiarch Luther Vuygel, the current high priest of the Imperial Cult and sitting member of the High Lords of Terra, was uncaringly teleported from the Great Chamber of the Senatorum Imperialis, one of the many under-hive hellholes where the most unfortunate or the most dangerous souls on Terra end up. There, down in the dark, his robes and symbols of office meant nothing to the gang that butchered and ate him. His death went unnoticed and unremarked on. Well, that isn't true. The gang did remark on how good he tasted.
The Unshackled kill teams sent to the Ecclessiarchal Palace move like ghosts through its gilded fortresses. They unmake the most influential priests and their servants with ease. It is not a war. It is a purging. It is a surgical strike lancing the cancer that is the Ecclesiarchy from the body of the Imperium. There are no bodies and no gore. One minute, they are in their offices, bed chambers, and gardens. The next, they are gone as if they were snatched up by the hand of a god. This continues all over the planet. The subtle eradication of the Imperial priesthood.
It is done this way because they have so ingrained themselves into the everyday lives of the citizenry that an open purge would lead to revolts and uprisings, which would lead to deaths that could have been avoided. This way, it will become a mystery that, combined with future events, will be considered divine intervention, and further down the line, when this boot of religious stagnation has been removed from mankind's neck, they will understand that it was just what had to be done. But for now, the First's Unshackled move in mysterious ways. Clearing his world of taint hour by hour. As he prepares to walk his children from this nightmare one step at a time.