The following is typical of the research methods I use when searching out obscure Luther quotes. In this first installment, I'd like to take a look at documentation.
To my knowledge, a complete professionally translated context of this citation is not available in English. With so many books available (particularly now with the Internet), it is quite possible it comes to light in English.
On page 122 of a self-published book, Martin Luther: Catholic Critical Analysis and Praise (2008), a Catholic apologist documents the quote as:
Bainton, 115; Carroll, 1; WA, Vol. VI, 347; EA, Vol. II, 107; PE, Vol. IV, 203; in reply to arguments of the Dominican Sylvester Prierias, Master of the Sacred Palace at Rome; On the Pope as an Infallible Teacher, or On the Papacy at Rome. Church historian Philip Schaff gives its Latin title as De juridica et irrefragabili veritate Romanae Ecclesiae Romanique Pontificis
I'd like to work through these references carefully. WA and EA refer to non-English editions of Luther's writings, and contain the context. Carroll, Bainton, and Schaff are secondary sources, and do not contain the actual context. If you can read Latin, the context is easily located. One can always attempt a web-page translator, but this method is typically prone to serious error.
PE Vol. IV, 203 refers to the Works of Martin Luther, otherwise known as the old Philadelphia six-volume English edition of Luther's works. PE Vol. IV, 203 though when checked contains only the title page "An Admonition to Peace: A Reply to the Twelve Articles of the Peasants in Swabia 1525. The citation though can be found in PE Volume III on page 203 in only the introductory material to a later treatise, An Earnest Exhortation For All Christians, Warning Them Against Insurrection and Rebellion, 1522. The introductory material was written by W.A. Lambert. Lambert cites the quote from a secondary source (Julius Köstlin), rather than a primary text. Thus PE Vol. III, IV, or any of the volumes in the PE set lack the context for the quote in question.
"In reply to arguments of the Dominican Sylvester Prierias, Master of the Sacred Palace at Rome" is an accurate statement, but this is not the title of the treatise from which the quote was taken.
By using italics, the Catholic apologist appears to make these two different titles refer to the same book: "On the Pope as an Infallible Teacher, or On the Papacy at Rome." Neither is the title of Luther's book, but rather is (are?) the title(s?) of a book by the Dominican Sylvester Prierias.
The actual name of Prierias's book is probably a combination of both titles above. This can be seen by the Schaff citation "De juridica et irrefragabili veritate Romanae Ecclesiae Romanique Pontificis." This translates roughly to "Of the lawful and irreformable truth of the Roman Church [ruled by] the Roman Pontiff." Schaff says this book was written by Sylvester Prierias, and republished by Luther. Here Schaff is in error. Luther did not republish this book, but another work by Prierias which was a shortened version, or an overview of De juridica et irrefragabili veritate Romanae Ecclesiae Romanique Pontificis.
Sylvester Prierias was writing a detailed treatise in favor of the power and authority of the Papacy (De juridica et irrefragabili veritate Romanae Ecclesiae Romanique Pontificis). Before the book went to print, Prierias published a shorter work which gave an overview of the arguments in his forthcoming book. He called this shorter work the Epitoma (1519).
Luther received the Epitoma by 1520. As a response to it, Luther republished it with his own annotations, introduction, and conclusion. Luther's Works explain this method is called,"Per contentionem"-
Per contentionem, a term used in rhetoric to describe the method of contrasting two arguments by printing them side by side. Luther used this method by reprinting Prierias’ polemic treatise, adding a foreword, a postscript, and marginal glosses. Cf. The Abridged Response to Martin Luther (Epitoma responsionis ad Martinum Luther) [LW 39 p. 172 (footnote 60)].
The exact title from which the quote comes from is Epitoma Responsionis ad Marinum Lutherum, and is found in WA, vol. 6, beginning on page 325 and ending on page 348. The quote appears on page 347 as part of Luther's postscript (Ad Lectorem). The Catholic apologist later cites secondary source Will Durant stating "Luther published, with furious notes, an Epitome..." but the Catholic apologist doesn't appear to realize this refers to the name of the treatise. Luther's Epitoma Responsionis ad Marinum Lutherum contains as one of its section headings "De juridica et irrefragabili veritate Romanae Ecclesiae Romanique Pontificis." This is actually the middle section of Luther's publication, and is the republished Epitoma of Prierias. Schaff doesn't seem to be aware of the difference between the two books, nor does the Catholic apologist.
The Catholic Encyclopedia though lists "Epitoma responsionis ad Lutherum (Perugia, 1519)" as the work of Prierias, not Luther. English identifications refer to it as the Epitome of His Response. Indeed, it appears both the work of Prierias and the republication by Luther use the same title. The Epitome was directly written against Luther, and was actually the third volume of a trilogy directed toward Luther, particularly Luther's treatise, Explanation of His Thirteenth Proposition Concerning the Power of the Pope [WA 2:183-240]. David Bagchi notes,
The first reaction to Luther's treatise was published in March 1520. This was Prierias's three-volume work, entitled, without undue modesty, Martin Luther's Erroneous Arguments Names, Exposed, Rejected, and Most Utterly Ground to Pieces [Errata et argumenta Martini Luteris recitata, decreta, repulsa et copiosissima trita]. The third volume was in fact an index to the previous two volumes, which Prierias entitled the Epitome of His Response- "for an index it is indeed rather long," he admitted, "but for an epitome, it is very short"- and which Luther reprinted with marginal comments and a foreward and an afterwotd in June of that year. [David V.N. Bagchi, Luther's Earliest Opponents (Minneapolis: Fortress Press, 1991), p. 48].
This analysis of documentation may seem like nitpicking. In actuality, it only documents the work I had to go through to determine where the quote came from. This helps determine what the context means.
In fairness, many sources do not helpfully explain where the quote comes from. I speculate Schaff probably worked from secondary sources as well with this citation, though he does provide the Latin text. In the Catholic apologist's case, his book on Luther is self-published, so providing the correct documentation is as easy as pulling up Microsoft Word on his computer and adding it.
Once the source is identified, next comes evaluating the context and historical setting. Stay tuned, as well as the way the quote has been interpreted.