Martin Luther was a German friar, priest and professor of theology who was a seminal figure in the Protestant Reformation. Luther taught that salvation and subsequently eternity in heaven is not earned by good deeds but is received only as a free gift of God's grace through faith in Jesus Christ as redeemer from sin and subsequently eternity in hell. His theology challenged the authority and office of the Pope by teaching that the Bible is the only source of divinely revealed knowledge from God and opposed sacerdotalism by considering all baptized Christians to be a holy priesthood. Those who identify with these, and all of Luther's wider teachings, are called Lutherans even though Luther insisted on Christian as the only acceptable name for individuals who professed Christ. Today, Lutheranism constitutes a major branch of Protestantism and overall Christianity with some 80 million adherents.