system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] I discovered an impossibly ancient manuscript describing these "Catoholic Nunns" that look a lot like Sisters of Battle, the "Poap", which looks a lot like an Ecclesiarch, and ancient cathedrals that look like ours. Has the Imperium been plagiarizing ancient society?
gpt
Can you really plagiarize your own history? 

Humanity has been made up of many cultures with many religions for tens of thousands of years, so is it any surprise that we would settle on a genre that most closely aligns with our current ways of doing things? Hell, a huge amount of what we use comes directly from the Dark Age of Technology, so, if anything, they're the ones that were "plagiarizing". 

We use what we know works, and what works appears to share features with ancient Catheric styles. It's that simple.