The way he's executed, Kellhus would be just a normal intelligent, if somewhat overly self-serving, person. So he might start a cult to prey on people who for some reason suspended their critical thinking, but it's not going to be even close to possible for him to sway another intelligent human.
It's actually one of the biggest problems I have with the series. Kellhus just doesn't sound all that convincing to me. Sure, he has some good points, plus some supernatural (and so far considered straight up impossible for a functioning human being) abilities, like learning languages in weeks. But he is tremendously overconfident and ignorant at the same time, and far from infallible. The way I see it, in our world, he wouldn't have more agency than another halfway smart man. And even in Earwa, he got so enormously lucky to even get to the Holy War.
The story needed him to fail in the Golden Room, but in reality he should've failed so much sooner.
To be fair, Kellhus was written by Bakker, so he can't be smarter than Bakker. You'll notice a suspicious connection between Bakker and most of his characters ...
The point is that Kellhus the character/place/plot-device is a super human intellect, so much so that he can do any skill - math, language, politics, martial arts - with ease. He baffles the minds of all others, out maneuvers every person or thing that gets in his way.
This Kellhus, who to all things are trivial, would find anything IRL equally trivial.
Note, Kellhus struggled a bit more with Conphas than anyone else - so I'd say the same is true: take any genius sociopath with massive amounts power and a huge following, and he'd struggle to overcome them... If we are talking about Kellhus as the Prince of Nothing.
Kellhus the infinitely powerful, who knows all and does all, would have no trouble with anything (except maybe if God(s) show up and take over his body).