Why was Martin Luther?s On the freedom of a Christian man (1520) controversial?

Project instructions:
Why was Martin Luther?s On the freedom of a Christian man (1520) controversial?

Readings – help
Alister E McGrath, Reformation thought: an introduction, 3rd edition, Oxford, Blackwell, 1999, especially pp. 86-91, 101-22 [in eReserve, Library]

Kolb, Robert, Martin Luther: Confessor of the Faith, especially chapter 5 [ebook ? available online through library)

Luther, Martin, Three Treatises, Philadelphia, Fortress Press, 1970 [1520], pp. 262-64 [in eReserve, Library].

Marius, Richard, Martin Luther: the Christian between God and death, Harvard University Press, 1999, especially chapter 15 [in eReserve, Library]

Mullett, Michael A Martin Luther, Routledge, London, 2004, especially chapter 5 [in eReserve, Library]

Wallace, The Long European Reformation [textbook], especially pp. 75-81.

Whitford, David M, Luther: A Guide for the Perplexed, especially chapter 2 [ebook ? available online through library]

Some useful general reference resources online include:
Cross, FL (ed) Concise Oxford dictionary of the Christian Church [accessible online through ?eResources? in library];
Encyclopedia Britannica Academic edition [access online through CSU library];
Hans Hillerbrand (ed.), Encyclopedia of the Reformation [accessible online through ?eResources? in library].
Please use correct referencing and page number in footnotes.