Quakers are people who belong to the Religious Society of Friends, a historically Protestant Christian set of denominations. Members refer to each other as Friends after John 15:14 in the Bible, and originally, others referred to them as Quakers because the founder of the movement, George Fox, told a judge to quake "before the authority of God". The Friends are generally united by a belief in each human's ability to be guided by the inward light to "make the witness of God" known to everyone. Quakers have traditionally professed a priesthood of all believers inspired by the First Epistle of Peter. They include those with evangelical, holiness, liberal, and traditional Quaker understandings of Christianity, as well as Nontheist Quakers. To differing extents, the Friends avoid creeds and hierarchical structures. In 2017, there were an estimated 377,557 adult Quakers, 49% of them in Africa followed by 22% in North America.