The Blessingists and the Shimmerists have a lot of conflict between them in the Pandemic Universe. Here is a quick rundown of where this hostility came from and the origins of the Blessingists.

Sunset Blessing was a highly influential Shimmerist in the early days of Shimmerism. While Shimmerism never had one singular leader, instead having many leaders, she had sway of a significant segment of the Shimmerist population, particularly in the southern US, but her ideas and methods were influential in other parts of the Shimmerist world.

In the days leading up to the Cataclysm of Riverview, Sunset Blessing discovered that her temporary transformation spell would always end up leaving humans with some magic. At this realization, she revised her goals as a Shimmerist, not to bring about a pony world, but a world where everyone could use magic. This could include magic-wielding humans.

Immediately after the Cataclysm of Riverview occurred, Sunset Blessing was facing arrest. Her last actions before this were to impart to Twilight Sparkle everything she knew about transformation magic and to author a letter to Shimmerist groups. She left this letter with her friend Number Crunch to pass on to those groups just moments before her arrest. This letter became formally known as the Blessing Letter.

The reaction to the Blessing Letter was mixed, with most Shimmerist groups rejecting it outright. However, there were a considerable number of Shimmerists based in Florida that took a favorable view of the ideas that Sunset Blessing had laid out in it, and they began their attempts to make reforms in Shimmerism. This was when the infighting within Shimmerism began.

Then a month after Sunset Blessing's arrest, she received a presidential pardon and made a televised speech to the nation. During this speech, she no longer talked of simple reform of Shimmerism but made a total renunciation of Shimmerism and denounced it as inherently speciest and evil. This immediately affected the already existing debates going on in Shimmerist communities. The traditional Shimmerists denounced those that wished to follow Sunset Blessing's reforms as traitors, and in the case of the more religious elements, they called them heretics. Sunset Blessing was likewise labeled a traitor and gained the title in Shimmerist groups of 'The Apostate'.

The reformers, faced with this bitter backlash and no longer able to carry on debates with their former comrades in Shimmerism, formally broke with Shimmerism, forming the Blessingists. The more religious among their number began to ascribe prophethood to Sunset Blessing. While the less religious still held her in high regard.

With this formal break, Blessingist numbers began to grow and spread to people that were never associated with Shimmerism and would have had nothing to do with Shimmerism if they could help it. This swelling of Blessingist numbers only served to enrage the Shimmerists more, and hostilities rose to a boiling point.

Blessingists are united behind the ideas of Sunset Blessing, but she has no involvement with the group. Her claim is she has no desire to be put on an undue pedestal or lead anyone, so she avoids making any public statements about the group. However, she continues with her vocal condemnation of Shimmerism, which only serves to continue to enrage the Shimmerists. The fact that she knew full transformation magic, the holy grail of magic for Shimmerists, and refused to release those secrets only deepened their hatred of her. To date, she and her family are regular targets for planned kidnappings, violence, and attempted blackmail, and the Blessingists are treated with outright contempt and open hostility by the Shimmerists.

On a related note: I've got to feel sorry for Sunset Shimmer. She acted out of desperation to try to save her world! The Shimmerists, if anything, are desperate to destroy their world. They're not worthy of Sunset Shimmer's name.

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Indeed