During the earliest history of Mankind, the God-Emperor was not seen as a divine figure but an enlightening figure that sought to bring the light of truth to Humanity in order to heal it from the works of false idols as well as demagogues. This became the basis of the entire Great Crusade, as the armies of the Emperor sought to bring the Imperial Truth to all of Mankind's lost colonies.
The noble goal was ultimately shattered during the turbulent times of the Horus Heresy when many of the Emperor's favoured sons, the Primarchs, turned against him. Thus began a war that became near mythical to the Imperium in later years as the Emperor's dreams were shattered. Despite horrific losses, the loyalists succeeded against the rebels but the victory cast the Emperor to an eternity imprisoned within the Golden Throne.
In the aftermath, the ideals of the Imperium had been shaken to their core, with many seeking faith to mend the scars they had suffered. Moreover, many now did not see the Emperor as a man but rather as a god that had sheltered as well as guided them.
On a thousand worlds, cults began to form that preached that the Emperor was a divine figure. These later turned into sects that warred against one another until only one remained. This one became the Temple of the Saviour Emperor and was the One Faith that formed the Ministorum. In the years that followed, the Imperium became an empire dominated by faith and the rise of the Temple of the Saviour Emperor made it such a powerful force that it gained seats amongst the High Lords of Terra. Its power began to dominate all facets of the Imperium, with the Ministorum's power being so great that only the Adeptus Astartes and the Adeptus Mechanicus were immune to its influence (though this is attributed more to the fact that no one was willing to enforce their submission).
In the time that followed, the Ministorum became the ruling force within the Imperium that commanded armies as well as fleets of warships with the power to make or unmake laws. As its power bloated, the Ecclesiarchy began to impose high tithes on worlds that sparked open rebellion amongst such colonies. This forced the Ministorum to send out its enforcers, the Frateris Templar, to quell the rebellions. These actions saw the rebel governors burned at the stake and entire planetary populations devastated for defying the Emperor's will.
During this age, a powerful warp storm engulfed known space that isolated many worlds from one another, which in turn descended into anarchy. The Ministorum's power grew even greater with their actions taking a toll on those few systems that were still accessible. At this point, the Ministorum challenged the Adeptus Administratum with the deadlock between the two only ending with the rise of Goge Vandire and the beginning of the Reign of Blood. Thus came the Age of Apostasy that only ended when Sebastian Thor formed the Confederation of Light that was considered heretical by the Temple of the Saviour Emperor. Thor was able to work with both the Astartes and the Mechanicus in ending the tyranny of Vandire, thus finally breaking the rule of the Temple of the Saviour Emperor. This saw six thousand years of rule by the cult being destroyed with the death of High Lord Vandire.
Following that time, Sebastian Thor became the head of the new Imperial Cult and became nominated as the next Ecclesiarch. His Confederation of Light became the new Ministorum that now declared the teachings of the Temple of the Saviour Emperor as heretical, thus driving its followers underground. The once formidable power of the Ministorum had been greatly reduced in scope and the Frateris Templar were disbanded in order to prevent the gross abuse of power that the Temple of the Saviour Emperor had demonstrated during their reign.
Whilst the great power that was once enjoyed by the Cardinals was gone and the Imperium had been reunited, the followers of the Temple of the Saviour Emperor managed to survive, waiting in the shadows and nursing their anger. These secret sects later formed the hidden heretical sect that became known as the Temple Tendency.
The core belief of the members of the Temple Tedency is that they are the "true" followers of the God-Emperor of Mankind. As such, they also believe the Ecclesiarchy to be heretics that have usurped their rightful place. Thus, all those that support them need to be purged with flame for their blasphemy. The devotees of the Temple Tendency are fanatical and very dangerous foes, when they believe that they are the ones fighting for the soul of the Emperor. The Temple's adherents believe that returning the Temple of the Saviour Emperor to dominance is necessary not only to allow Humanity to repent for its sins, but save itself. Whilst a large faction within the Imperium, the Temple Tendency aims more for survival and secrecy rather than openly challenging the Ministorum.