In Sonny 1, when you fight against Baron, Galiant is on your side.
Was Galiant one of the cultists? ...The ZPCI don't look like the kind of group to be having a lot of armor-clad sword wielding holy warriors on their side.
If Galiant was one of the cultists, what was he doing gathering "reinforcements" from the ZPCI? Also, fighting the Baron in the first place.
Baron is supposedly some kind of zombie manipulating super zombie. The cultists said that the baron lied to him (meaning they didn't know he was a zombie). In Sonny 1 he somehow caused pain to Sonny and Veradux's heads (zombie manipulation i would guess).
THat said, Paladin is definitely in the ZPCI, first of all, he is wearing ZPCI armor. Secondly, in the level before, the ZPCI sniper states that tthey would meet up with the Paladin later to help fight the Baron.