Luther taught that salvation and subsequently eternal life 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. 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 or Evangelical as the only acceptable names for individuals who professed Christ.
Since he didn't believe that salvation was acquired through good deeds, how was it achieved? What did Martin Luther believe achieved God's Grace? Whose authority did Luther's theology oppose? What did Luther think was the only source of knowledge of God? To Luther, what were all baptized Christians considered to be?