How many people in 40k actually know the names of the chaos gods, either as we know them, khorne, nurgle etc or there 'true' names in dark tongue of Kharneth, Nurgleth, Tzeeneth, Slaaneth.
In a Gaunts Ghosts book seige of Vervunhive Gaunt names them, and here's pretty unique in that he could have been a warmaster for a crusade and not just a Colonel-Commissar.
In Angel Exterminatus the first time the Emperors Children say the name Slaanesh on a crone world a lightning storm erupts and there's an earthquake like its a really big deal to utter the true name.
Lowly chaos cultistis would have no idea of their names right, perhaps even figures as high as space marines wouldn't know since its restricted knowledge and the Imperium is all about hiding information and censoring it. Yet in some games like Darktide 2 random enemies will scream 'for grandfather nurgle' and stuff
https://youtu.be/t6zhuFAoC6Y for some ambience for our discussion of hidden knowledge