No major institution anywhere systematically delegitimated the institution of slavery anywhere until the Christians - especially Protestants, and especially British MProtestants - did so. Essentially the Christians of the American South were uniquely situated to be the last to “get" - that is, accept - the word.Published argumentation defending slavery imposed by Christians never occurred anywhere except in the American South. For the simple reason that that was the only place where slavery by Christians both was entrenched and was under Christian moral attack.
There is a very real argument to be made that American Protestants did just as much, perhaps more (and did it sooner) to end slavery than that of British Protestants. At least prior to independence anyways.
Guys like Anthony Benezet, John Woolman, there were a lot of these guys and they preceded Granville Sharp by at least a decade and preceded Wilberforce by probably a half century.
I would say that their hard work paid off, considering that at least 6 states abolished slavery in the nation’s first several decades.