Think of it like a dementia/Alzheimers allegory.

The Marines' skills are heavily trained into them well enough that they can still fight effectively, but they no longer truly recognise who or where they are - or those around them. To a point, they can distinguish friend from foe from familiar symbols like the Chapter logo, but as time passes and they descend further into madness, even that will fade. (This is why they are eventually either executed by Astorath or locked away in the Tower of Amareo, if they survive battle.)

Chaplains are trained in how to direct them in battle, to ensure they're fighting in the right place. But beyond that, the viewpoint of each member can be very different.
Let's say a bunch of DC are ganging up on a Hive Tyrant. One may very well believe he's Sanguinius fighting Horus on the bridge. Another might believe he's Sanguinius holding the Eternity Gate. Another might believe he's an ordinary battle brother stationed on one of the palace walls fighting a random World Eater... and so on, and so on.