Emperor could have solved a lot of his problem primarchs with a simple conversation.
"So. Lorgar. I know you see me as God and are genetically predisposed to the notions of the divine and sacred. I got you man. But, listen, I have a big plan here, and it requires us to be atheists right now. I'll explain later, but, just have faith in me buddy, okay? I'm not a god but it's cool if you wanna obey me like one. Just don't go tellin people bro. Conquer some planets for me for a while and it'll all be clear pretty soon."
"Curze, buddy, pal, you need to chill out. Also, have you considered law enforcement isn't a one-man job? I mean real nice what you did with Nostromo, no kidding, I'm a huge fan, I'm proud of you. But maybe hire some deputies for when you're not around. Also: those prophetic visions? They're wrong sometimes. It's just potential future. The future isn't real per se. You can change it to be what you want, not what you fear. Anyway, maybe try being less of a doomer for a bit, you're scaring Rogal Dorn."