Their purpose is to create or find a vessel by which they might birth Samael into this plane of existence. The Hope House was one such means as was simply breeding their women selectively. The female children then apparently gained some psychic gifts of varying power from Samael, most notably Alessa Gillespie and Claudia Wolf. The concrete tower outside the perimeter of Wish House may have been used to aid them in their quest, perhaps calling upon Samael to lend his power. Dahlia Gillespie, mother of Alessa, along with Dr. Michael Kaufmann, botched a ceremony performed upon Alessa when the girl was seven years old, and burned down half of Silent Hill with it. Half of Alessa's soul went into the creation of an infant girl, who was found by the side of the road by Harry Mason and his wife.
After having been nearly eliminated at the hands of Harry Mason, the Order laid low, keeping under the radar of anyone who might act against them until James Sunderland arrived in the town. He performed the Crimson Ceremony, calling Samael back to power, and he resurrected his wife along with the Order itself. It is believed that he took up leadership of the Order after that.
Years later, Father Vincent was at the helm of the Order. It is not known what sort of relationship that he had to anyone else in the Order that he should rise to such a position (though it is quite possible that he is James and Mary's son actually). Claudia Wolf was a prominent member as was her father, Leonard, though he had been committed to Brookhaven Hospital due to stabbing someone over religious matters.
Vincent had been embezzling funds from the Order's coffers, and Claudia herself sought to raise God with a fanatical devotion. She hired Douglas Cartwright to find Harry Mason and thus the girl she claimed had been kidnapped from Silent Hill seventeen years ago, going so far as to have a creature called the Missionary murder Harry in order to engender hatred in Heather's heart. In the process of attempting to bring Samael into this world, Claudia stabbed and killed Vincent, and she tried to salvage the effort after Heather used the tablet of aglaophotis in the pendant her father had given her to expel the fetus from her body. Heather then killed Claudia and left the town with Douglas.
Afterwards, it appears that the Order may not have taken as large a hit as it might seem. It was intimated that there were other members of the Order still, and any one of them could have taken over leadership. The quest to bring Samael into this world is not on hold, simply set back to the point where they have to create a new vessel. The process was started some years ago as a backup plan while they were still searching for Harry and Heather.
Walter Sullivan, a serial killer, was born in Room 302 of the South Ashfield Heights apartment building which is owned by Frank Sunderland, most likely a relative of James and a member of the Order himself. He was abandoned by his parents, probably members of the Order themselves, and was taken to the Wish House. At the age of six, he was taken back to Ashfield and shown the apartment where he was born. He perceived the apartment as his actual mother. He habitually returned to the apartment, eventually to be run off by the tenants of the apartment building.
The ritual of the Twenty-One Sacraments brings about the return of the Holy Mother (a vessel for Samael), but Walter believed that it would bring back his own mother. Walter began the Sacraments with ten murders, including a priest of Valtiel, and cut out the hearts of his victims then carving a series of numbers on their bodies beginning with 01121 and ending in 10121. Once arrested, Walter committed suicide in his cell by stabbing a spoon two inches into his neck and severing the carotid artery. "11121" was carved into the soles of his feet.
Two years after his death, another body was found with "12121" carved into it, raising old memories of Walter's killings. It was dismissed as a copycat crime since the heart was not missing and Walter had committed suicide much earlier.
Through means of a nightmare world, Walter was able to succeed in killing up to the nineteenth of the twenty-one required victims, leaving only two left to complete the series and bring about the descent of the Holy Mother. It is not known whether or not the same person must complete all twenty-one murders in order to complete the Twenty-One Sacraments, something that Order must now contemplate for their own ends.