The Quaker use of "thou"
The Quakers are a religious denomination with Christian origin, founded in England in the 17th century. Their official name is "Society of Friends". They were dissatisfied with the existing Christian denominations; the name Quaker derives from the verb to quake.
They adhered consequently to the use of "thou" as pronoun for the 2nd pers. sing. They supported the distinction between "thou" (singular) and "you" (plural) made in the Bible.
As they continued to use "thou" even when this pronoun became more and more obsolete, "thou" became a characteristic feature of the Quakers.
This can be seen for example in the entry made by Samuel Pepys in his Diary (11.1. 1663/4):
Read the full entry here.
The adherence to "thou" evoked strong and sometimes violent disapproval from the family and the public (cf. Finkensteadt, 177). In his Journal, George Fox (1624-91) who is regarded as founder of the Quakers, describes several times how people, especially those of higher social rank, reacted with hostility when he addressed them with "thou" (cf. Barber, 211).
By the way the Quakers used more and more "thee" insteadt of "thou" in subject position.