Their roots go back to 1968, when four schoolgirls from the small Spanish town of El Palmar de Troya claimed they saw an apparition of the Virgin Mary. The local bishop countered with, "Nah, you probably didn't," but believers emerged anyway -- chief among them an initially skeptical Clemente Dominguez y Gomez, a failed priest turned office clerk. Dominguez, among others, claimed that he also received visions when he visited the site of the "miracle." But then Dominguez added that Mary had instructed him to rid the Catholic Church of "heresy and progressivism," even as those schoolgirls who kicked this all off retorted with, "Uh, you're weird, please don't speak for us."
By this point, tens of thousands of pilgrims had visited the site, and even though the Church had declared the whole affair to be superstitious nonsense, some visitors supported Dominguez. Never mind how absurd context was; these were people who were missing something in their lives, and Dominguez gave it to them.
While the girls returned to their regular lives, Dominguez officially launched his own religious order and hit the road to preach. Now, countless people across America are doing precisely that as you read this, but in 1975, Dominguez (now dubbed Father Fernando) actually found an archbishop who was willing to make him and his flock official. And because this story wasn't already ridiculous enough, that archbishop was Ngo Dinh Thuc, brother of the guy who ran South Vietnam as an election-rigging, Buddhist-oppressing dictator before he got a bullet in the head for his trouble.
Thuc was later excommunicated for being a trouble-making weirdo, and his ordinations were considered about as valid as when a football team signs a Make-A-Wish kid to a one-day contract. But remember, Dominguez's whole shtick was that the Church was a misguided den of Freemasons and communists who committed unspeakable evils like, gasp, slightly modernizing Mass. He claimed that Pope Paul VI was being manipulated by malicious Vatican cardinals.
Then in 1976, after he lost both his eyes in what must have been one hell of a car accident, Dominguez claimed that Jesus came to him in a vision and declared that he would be the next pope. A great pope. This is a man who, in less than a decade, went from "Eh, I guess I'll go see what this supposed miracle thing is all about" to "I am destined to be the most important religious figure on the planet." And so, when Paul VI died in 1978, Dominguez declared himself Pope Gregory XVII.