There’s a section which may be of interest on the Plymouth Brethren in Christian Sects in the Nineteenth Century by Caroline Frances Cornwallis, beginning on page 84, a work I noticed was added to Project Gutenberg last month.

“the great feature of this sect, for so notwithstanding their protest, I must call these “Brethren,” is a degree of self approbation and uncharity for others, which, to say the least, is not what Christ taught. “No sect,” says Rust, “is more Sectarian, and none more separate from Christians of all denominations than “The Plymouth Brethren.” The Church of Rome they consider “bad.” The Church of England “bad.” “A popish priest and a parish priest, both bad;” “but infinitely worse,” says one of the Brethren (a Captain Hall), “is a people’s preacher.” They occasionally indulge in what they term “biting jests and sarcastic raillery,” of the ministers of our church, and of those who differ from them, which evince but little of the meek and peaceable spirit of the Gospel; for, as Lord Bacon has well observed, “to intermix Scripture with scurrility in one sentence;—the majesty of religion and the contempt and deformity of things ridiculous,—is a thing far from the reverence of a devout Christian, and hardly becoming the honest regard of a sober man.” If I have appeared to speak harshly of this sect, it is because they seem to me to have abandoned so much of the spirit of the Gospel. “If the tenets of the Plymouth Brethren be consistent with themselves,” observes Mr. Rust, “they necessarily withdraw them from all society, and every existing form of Christianity, shutting them out from all co-operation with the holy and benevolent, for the relief and blessing of their poor or sinful fellow creatures, making it sinful to fulfil the duties of a subject, a citizen, &c.” But I hope and believe that these tenets must be and are counteracted by the instinctive love of our kind, which for the benefit of the world God has implanted in man. The human race is so essentially social that they who endeavour to dissociate mankind, stand in much the same situation as he would do who should hope to dam up the ocean. It is in fact to these silent tendencies of human nature, whose force we never know till we attempt to check them, that we owe much of the innocuousness of false or overstrained opinions: the reason is deluded, but the feelings which the Creator has made a part of our very being, generally correct the false argument; and the man, if not previously corrupted by vice, acts right though he argues wrong.”

Of course, there’s quite a bit about the Plymouth Brethren in both Aleister Crowley’s Confessions and also in the introductory materials to The World’s Tragedy. You can find these doing a site search for “Plymouth Brethren“.