You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Martin_Luther

Context:
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.

Question:
What did Luther think was the only source of knowledge of God?

Bible