Who are the wall cultists? In episode 14 of Attack of Titan season 1, there's a man in a white robe whom Eren calls him the wall cultist.

Who is he?
For the people inside the walls, these very walls constitute a wonder. No one knows where they came from, only that they protect the residents from a mortal thread. And the walls did a pretty good job for about 100 years.

Now, when people cannot understand something, often they tend to interpret it as a result of divine intervention, especially in times were science isn't as advanced as today. Wall cultists are those who think of the walls as holy. In the time shown in the anime, they got increased influence over the population.

Additionally:


   Science wasn't exactly encouraged within the walls.