The Eldian people in Attack on Titan anime initially had no concept of religion or belief. It was only after the appearance of Ymir Fritz’s supernatural power that the concept of worship emerged. The power of titan shifters was held in high regard and seen as demon power by the Marley race. The Eldians began to worship and idolize Ymir and her power. This occurred in accordance with the story of Ymir which was once idolized like a deity when she was with a group of religious people in Marley, around 60 years ago before she became a pure titan and devoured Marcel to become the titan shifter.
After the Eldian people migrated to Paradis Island, King Karl Fritz erased the memory of Eldian history that was linked to titan power. The government then built a church for family rituals, one of which was to inherit the power of titan shifters. On the other hand, the Eldian people on Paradis Island also had a group of wall worshipers led by the trusted and authoritative figure, Pastor Nick. He tried to eliminate Eren, the titan shifter. The wall worshiper group, however, was engineered solely to protect the kingdom’s secret about titan power. The government wanted to trick their people and cover up their malice with a veil of belief. It was a collusion between the ruler and the religious group.
Pastor Nick was later arrested by the Scout Regiment as he was suspected to know something about titan power. Unfortunately, before they could dig deeper, Pastor Nick was killed by the Military Police who were ordered by the false king. When the falsehood of the religious authority and the ruling government were uncovered, the Eldian people on Paradis Island were left confused. Or perhaps, most of them did not consider the metaphysical and philosophical implications, except for a small minority. They were too busy waging wars.
When one of the Marley prisoners, Onyankopon, arrived in Paradis Island accompanied by Yelena, Zeke’s subordinate, he helped Armin and his friends develop technology that was deemed to be 100 years behind. Onyankopon stayed for a long time in Paradis Island, and thus, there were numerous interactions between him and the Eldian people. One day, Armin asked Onyankopon about who created humans. Onyankopon, who came from the outside and had the concept of God, answered, “He gave the Founding Titan power to Ymir, God.”
It is yet to be known what will be the next belief system of the Eldian people on Paradis Island. While Eren plans to eradicate all races outside Paradis Island, every philosophical thought regarding creation and knowledge will also be destroyed.