>>96872296
Marines are ordo militaris in particular
Imperial Knights are regular nobles
Rogue Traders are Conquistadors
Inquisition are, duh, an Inquisition
Ecclesiarchy are just a Church
Admech are monks and thus the educated caste
SoB are nuns

And then there is Imperial Guard, which unlike everyone else in Imperium isn't based on medieval counterpart. Instead they are inspired on WW1, WW2 and modern armies and as such do not fit the Imperium and the setting itself