The Church of the Walls (壁教 Wōrukyō?) is a religious organization that considers the Walls to be sacred, with a lot of religious and political power within them. Some of the church's members possess knowledge about the Walls that is hidden from the general public.
After the truth about the Titans is revealed to the citizens of Paradis Island (and the later destruction of the Walls due to 'The Rumbling'), the Church of the Walls is assumed to be most likely disbanded. However, one key member is still seen serving the new Royal Government under Historia Reiss.
The church worships the Walls over everything else, believing them to be sacred and untouchable. They worship Wall Rose, Sheena, and Maria, and think that it is their responsibility to protect the Walls from any "impurities," like humans, rails, or Wall-mounted artillery. They have been shown to be uncannily loyal, to the point where they would sacrifice their own lives if it meant the protection of the church's secrets. In addition, they believe the Walls were a gift from God bestowed upon mankind in order to defend them from the Titans.
Members of the church are referred to as "Wallists." Among them there are a variety of different people from different places. There are also ministers who represent the Church of the Walls. An unknown number of ministers are involved in deep secrets regarding the Walls, which also includes information regarding the Titans inside the Walls and possibly even the mystery of the Titans. The only notable member of the church seen thus far is Minister Nick, who is eventually murdered by the First Interior Squad, after seemingly revealing some of its secrets.
The Church of the Walls is directly involved with the Royal Government, as seen when one of the ministers attends a meeting with the fake King. Traitors are treated without any mercy. When Nick decided to help the Survey Corps, the Interior branch of the Military Police Brigade was sent after him, which caused him to be tortured and murdered.