Religion

Religion is a concept that appears in the Fullmetal Alchemist series. It refers to a system of belief practiced by a group of people, usually involving an object of worship.

Letoism
Letoism was a religion created by Cornello to deceive and control the people of Reole. By performing supposed "miracles" through the use of a Philosopher's Stone, Cornello convinced the Reolans that he was a prophet of the sun god Leto and that Leto would grant miracles, such as the resurrection of Rose's lover, as a reward for their service. As a result, Rose became a devout follower of Letoism and only realized Cornello's deception when the Elric Brothers arrived in search of the Stone. Letoism was kept alive by the Homunculi for some time afterwards through Envy's impersonation of Father Cornello, who used the religion to create violence in Reole by pitting the Letoist believers against the non-believers so as to use Reole as a Crest of Blood for the Nationwide Transmutation Circle. After the defeat of Father, the reconstruction of Reole, and the deaths of all his Homunculi, it can be assumed that Letoism has died out, with only a few believers remaining.

Ishavalan Religion

The Ishvalans were known to worship a single creator deity named Ishvala. Ishvalan religion was known to forbid alchemy, and had been passed down through the generations by prophets such as Logue Lowe, proving a much greater endurance than Cornello's sham religion. Ishvalan religion also suggested an afterlife in which deceased people would meet the Ishvalan God, as Scar would often claim that people he killed would be reunited with God in death. After the Ishvalan genocide, very few practictioners of this religion remained alive, though through the Ishvalan survivors, it did continue to exist after the The Promised Day and subsequent defeat of Father. In the manga only, the Ishvalan religion's promise of afterlife appears to be correct: Hohenheim was reunited with Trisha Elric after his death and spoke to her excitedly about their sons' defeat of Father and the method by which Ed retrieved Alphonse's body.

The Truth and the Eye of God

The Truth would claim to be "God" when encountered by alchemists who had attempted to tamper with alchemy's most fundamental laws. Father, intending to "swallow" God and become all-powerful with the use of the Nationwide Transmutation Circle, appears to have been referring to the Eye of God (a metaphysical entity beyond The Gate), and believed that doing so would grant him unlimited knowledge. While Father was successful in initially swallowing "God," he was defeated by Edward Elric and his allies and forced back into the Gate by The Truth for his arrogance. As the Truth also introduces himself as the Universe and all things in it, it appears this God is not a deity in a traditional sense but rather the embodiment of all of existence and its laws. No known individuals worship the Truth or the Eye of God.

2003 Anime
Christianity

In the 2003 Anime, the Truth and the Eye of God do not appear and are replaced by The Gate Children; Letoism and Ishvalan Religion play largely the same roles as they do in the manga and Brotherhood. However, Christianity is known to exist as well (or at least have existed at some point), whereas according to Hiromu Arakawa, it explicitly does not in the manga (and presumably Brotherhood). The Flamel, an alchemical symbol detached from any religion in the manga but derived from Christianity in the 2003 series and real world, appears on Edward Elric's jacket, Alphonse Elric's armor, and other locations. It is known that Christians worshipped a single God taught to them by their prophet Jesus Christ but largely died out due to their intolerance. Dante takes Rose to a Christian Church underneath her lair, where Christians had constructed a secret city, and Clara impersonates a Christian Nun, suggesting the Religion is still alive (or at least has influenced aspects of Amestrian society). It is unknown if Christianity is practiced on the other side of the Gate, but likely is, as it is practiced in the real world and was the dominant faith in Weimar Germany.