The Universal Church of Truth is a religious empire in the Marvel Comics universe, composed of diverse alien species from across the entire universe. Its seat of government was Sirus X, also called Homeworld, in the Hercules star cluster. The Church's purpose is to convert the entire universe to its doctrines and thereby expanding the empire. They are the collective arch-enemies of the Guardians of the galaxy.
The Original Church of Truth
The head of the Church and object of its worship was Adam Warlock's evil self, the Magus. The assistant to the Church leader is the Matriarch. The Guardians of the Galaxy later encountered the Church, where they learned it was governed by the Protégé and controlled a large portion of the Milky Way.
Doctrine is enforced through the Church's Black Knights, and matters of law are subject to a Grand Inquisitor. The Church considers all non-humanoids to be their enemy, not being in the form of their god. This includes certain 'undesirable' humanoids such as trolls. Enemies of the Church are forced into slave labor, held prisoner, used to power the Church or terminated. Trials are sometimes held, although they are corrupt. Sentences are usually rehabilitation, which is just the Church using will breaking drugs and mental projections to brainwash the subject.
The Church, led by the Magus, was eventually erased from existence when Adam Warlock traveled his own timeline and killed himself before he could become the Magus. This would not be the end of the Church forever though.
The Return of the Church
The Church somehow resumed activities and grew in size exponentially. Now led by the Matriarch from their new homeworld of Sacrosanct, the Church built 'faith engines' from the power of their worshipers. The faith engines powered everything from their colony worlds, their Templeships, and even their new elite guard, the Cardinals.
After one of their Templeships was nearly destroyed, the Matriarch sent Cardinal Raker and his Cardinals after the attackers: The Guardians of the Galaxy. At the Matriarch's request, a blood sample was taken from one of the attackers and she learned that he was none other than the one they worshiped, Adam Warlock.
Having recently left the Guardians of the Galaxy after the revelation of Starlord's deception, Warlock took Gamora with him and began searching out the Church of Truth. During one of their encounters with the Church's Cardinals, Warlock revealed that he knew that the Church played a role in the coming cataclysm. He also revealed that he had decided he could best prevent it, by leading the Church.
Adam revealed himself to the Matriarch and told her that he is the one they worship and he will prove it by saving the universe. This perplexed the Matriarch, who believed she had the real Warlock in a cocoon in her Church. Adam was able to save the universe, but not without personal cost to himself.
In order to stop a rip in space-time known as the Fault from consuming all of reality, he overlapped an unused future with the current reality. The future he used was the Magus future, and in doing so became the Magus. (Overlapping time with this future essentially made Adam into the Magus for several months, unknown to him, and explains how the Church came back into existence.) The Guardians of the Galaxy were informed by Kang the Conquerer that all futures were becoming Magus futures and he had to be stopped.
With the help of a Cosmic Cube, Starlord was able to quell the Magus inside Adam, but only temporarily. To stop the Magus, Starlord killed Adam at his own request. The death of Adam and the Magus caused many Cardinals to go insane with grief causing a chain reaction in the belief fonts that power many of the nearby Templeships, causing them to detonate. However, this would not be the end of the Church.
After having took interest in a creature that emerged from the Fault that was contained inside Moondragon, it is revealed that the Magus is not dead. He had created the illusion of his defeat so he could continue his evil plans. After the Guardians of the Galaxy stopped their plans with their new "god", they focused on releasing the contents of their cocoon. Much to their surprise, it was not who they believed to be inside, but it was in fact the Mad Titan Thanos the Mad Titan. When he awakes, the Magus and the other Church officials escape while the remainder of the population is killed by their angry guest.
It is revealed that the Magus plans to allow the inhabitants of the Cancerverse into the universe, resulting in the destruction of Death and the end of the universe as it was. The Magus uses dozens of planets converted to the religion as a catalyst to open the Fault further, igniting them into massive explosions and killing billions upon billions of Church believers.
The Faction War
After the death of the Magus during the Thanos Imperative, separatists from the Church calling themselves the Western Spiral Order waged war over control of the belief fonts and the billions of worshipers. Matriarch Aqwen set up a sect on Earth called the Life Science Institute in Colorado to facilitate the rebirth of the Magus and reunite the Church.
After the Annihilators captured Aqwen and learned of their plot, they headed to Earth to stop the rebirth. The new Matriarch was able to release the Magus from his cocoon, although he was still in a child-like form. In order to protect himself, he deviously transferred his essence into human children. He then went on to use a belief font to take control of millions of American citizens.
In the 31st century, the Universal Church of Truth has long been a dominant force in the universe, enslaving and converting races across the galaxy for centuries passed. They raised a cosmic entity known as Protégé to be their one true god. Malevolence, the evil daughter of Mephisto, won the battle with Aleta of the Guardians of the Galaxy to be his Matriarch. She would begin their plan to corrupt Protege for their own devious purposes, as he had the ability to learn the powers of anyone that used them in front of him.