The Western Church is a religious organization in Fire Emblem: Three Houses. It is an offshoot of the Church of Seiros, with its headquarters located on the border between the Holy Kingdom of Faerghus and the Adrestian Empire. Originally, the Western Church was a regional branch of the Church of Seiros that oversaw western Fódlan, but at some point, its beliefs diverged and it became a separate denomination. Although the Western Church still worships the Goddess, it considers the main Church to be heretical and Archbishop Rhea to be an apostate, with extremist members and various leadership willing to incite rebellions and attempt to murder her on multiple occasions. It is believed by some that their bishop promotes such beliefs at least partly out of a personal desire to have authority over the rest of the Church. Further, secret articles in Abyss state that a number of them also came to believe that the Knights of Seiros were the ones who caused the Tragedy of Duscur, further inciting their belief that the Central Church was wicked. Their differences in belief and doctrine are mostly not stated, except that they advocate for more extreme separation of class, race and culture, and oppose the mingling that Garreg Mach Monastery allows.
After years of disputes with the Church of Seiros, the Western Church finally escalated tensions into open conflict in Imperial Year 1180, manipulating Lord Lonato to futile rebellion in order to reveal a fake plan to assassinate Rhea and thus distract from their real plan to break into the Holy Mausoleum and steal Saint Seiros' bones. Unknown to them they themselves had been manipulated into this plan by "those who slither in the dark" for their own mysterious ends. While the plan failed, the Western Church's diversionary attack on the Goddess Tower is noted by Manuela as having injured and killed a number of students. Most of the attackers were killed in battle, and the members who were captured were soon executed for their various crimes. The Western Church's corrupt leadership was subsequently purged in a series of missions by the Knights of Seiros, with Rhea and Byleth personally going to find and execute their bishop. They eventually reformed under a new bishop, but a radical faction emerged to resist the new order by moving to violently seize a sacred cemetery on the Rhodos Coast. The Church of Seiros dispatched Seteth to reclaim the cemetery, but Flayn insisted on accompanying him with Byleth out of fear for his safety. The radicals were routed, and it was hoped any who remained would see reason and cease their violence.