This article is from the The Religious Society of Friends FAQ, by Marc Mengel firstname.lastname@example.org with numerous contributions by others.
Among early Quakers it was traditional to call everyone and
anyone thee and thou, including royalty and church officials,
who were to be referred to in the plural in deference to their
official Holier than Thou position. This practice continued
for some time after English speakers started calling *everyone*
"you" rather than "thou."
Only a few (usually older) Friends use thee and thou anymore.