Quakers are people who belong to the Religious Society of Friends, a historically Protestant Christian set of denominations. Members of these movements are generally united by a belief in each human's ability to be guided by the inward light, "answering that of God in every one". Friends 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.