Decree Passive

So after the Age of Apostasy, the Imperium issued the Decree Passive, that said the Ecclesiarchy could no longer have "men under arms" to prevent them from taking over the Imperium again.

In strange amount of rules lawyering, Thor said that the Sisters of Battle should be exempt, since they were women under arms. Everyone agreed to this for some reason. As the 8th Edition version of Codex: Adepta Sororitas in 2019 explained, Thor vouched for them.

On top of that, the Ecclesiarchy has other ways to get around the Decree:

• Crusaders (Inquisitors like using them so they get a pass)
• Frateris Militia
• Shrine world PDFs
• Any random mob of pitch-fork and torch bearing citizens a local priest can drum up
• Cult of the Redemption (though they're considered unsanctioned extremists by the rest of the Ecclesiarchy, and that's saying something)

In short, the Decree is just an excuse for the Church to have an order of sexy battle nuns.