HistoryCannonwalker, AwesomeA321 and Captain_Naughty all started out as members of a large man-made island, accompanied by many of their friends. However, as Cannonwalker and Naughty started to grow discontent with life on the island they decided to sail away and form their own separate societies; Cannonwalker formed the Brotherhood, while Naughty formed Windforth.
While Windforth was a Nordic-themed village in the extreme hills biome, the Brotherhood was an underground base built on a very small island (too small to even build a house on). AwesomeA, one of Cannonwalker's friends, showed interest in Cannonwalker's new society and Cannonwalker invited him to join it. Captain_Naughty would also join later on, resulting in the discovery of an abandoned mineshaft underneath the base.
Although the base was originally abandoned (to some extent) after the members migrated to Elitecraft to join the Templars the group's disillusionment with life in Elitecraft caused them to migrate back on June 29, 2014, bringing along with them new recruit Ned_Edgewalker.
Eventually, BaronVonShush, another member of the Brotherhood, took to creating specially enchanted armor, acquiring already enchanted pieces from the island's mob grinder. In time, however, the Brotherhood began running out of room to store armor pieces that hadn't been merged yet; to rectify the problem, BaronVonShush decided to enchant books instead so that he could get better enchantments quicker. In addition, he wanted to build a library on the island. Sadly, most of the cows in the Minedlands had been killed off or sealed away where Baron couldn't get to them. This made it extremely difficult to find a source of leather for books.
First, Baron sailed off the coast of the island in hopes of finding a new continent; if he was lucky, the cows there wouldn't have been killed off. However, after hours of traveling, Baron gave up and returned. He set off again, this time for a nearby island that hadn't been touched yet; again, he found nothing. Baron proceeded to search the main continent of the Minedlands, and after several more hours of exploring the entire thing, he came across a small herd that had not yet been killed. Immediately, he claimed a small area and created a small building; inside, he planted wheat and carrots so that he could feed himself and his newly-captured cows. For an extended period of time, Baron killed the offspring of the cows for leather, but he also helped raise them into adulthood and released them into the wilderness, hoping to repopulate the continent.
The name "Brotherhood of the Golden Pants" was thought one night on the island. Cannonwalker had been thinking of leaving the island and so happened to kill a zombie with golden armor. Cannonwalker received the golden armor pants it was wearing and decided that "this is what my new society will be about." Why Cannonwalker is so fond of the golden pants, nobody knows.