Decree Passive

the Decree Passive is a decree promulgated in 288.M36 by the High Lords of Terra in the aftermath of the Age of Apostasy. In essence, it forbids the Ecclesiarchy to "gather, train, promote, sustain or in any way command any force of men under arms", to prevent another occurence like the Age of Apostasy, where High Lord Goge Vandire became at the same time head of the Senatorum Imperialis and the Ecclesiarchy, and could keep any Imperial citizen in his thrall with the threat of branding him heretic and unleashing billion upon billion of Frateris Templar upon any who disobeyed him.

History
Goge Vandire, the 361st Lord of the Administratum, is one of the most reviled human beings in all the history of the Imperium. To his credit, he did manage to break the stranglehold the Ecclesiarchy had over the Imperium in the late M35, and engineered the election of an incompetent Ecclesiarch, Paulis the Third, thanks to contacts he had within the Officio Assassinorum, foremost amongst them the traitorous Tziz Jarek. Capitalizing on Paulis' inefficience, he managed to wrest control of the Imperium from the Ecclesiarchy at first, and ultimately installed himself at the head of the Ecclesiarchy after getting rid of the most proeminent and troublesome Cardinals.

Vandire, however, soon proved to be even worse: he was a paranoid megalomaniac who believed everyone was out to take his place, and he did not hesitate to use Callidus Assassins to eliminate any individual who dared speak against him, or unleash billions of faithful Frateris Templar who fervently believed they were doing the Emperor's work, upon any planet who dared resist him. Vandire's exactions lasted for 70 years until Sebastian Thor started and successfully led a revolt against the madman.

In the aftermath of the bloody campaings that led to Vandire's demise, Sebastian Thor oversaw the implementation of some reforms eerily similar to those Roboute Guilliman implemented after the Horus Heresy, to prevent any future possibility of a single individual gathering enough power to threaten the very stability of the Imperium ever again. Amongst the many reforms are the cration of the Ordo Hereticus and the Ordo Sicarius, to control respectively the "high and mighty" within the Imperium and the Officio Assassinorum, but central to those reforms is the Decree Passive, wich is a forbiddance under pain of immediate execution for any member of the Ecclesiarchy to retain any men-at-arms under his control. The only tolerated exception to the Degree Passive are the Adepta Sororitas.

in M41, the Decree Passive is still being enforced, but less zealously: bands of Frateris Militia are allowed to gather in the shrines and holy places of the Ecclesiarchy, and crusades of faithful zealots such as the Redemptionists have been tolerated as well, albeit under strict monitoring by the Ordo Hereticus. Cardinals and high ranking Ecclesiarchy members are still not allowed more than a few Crusader bodyguards in day-to-day life, however the Decree Passive can be circumvented in dire tidings by having an official of the Departmento Munitorum rubber-stamp an approval to conscript the Frateris Militia as Imperial Guard auxiliaries for the duration of an emergency, allowing the Ecclesiarchy to equip and lead the faithful without risks of being executed even if they are victorious.