It's often claimed (mostly by Roman Catholics) that Luther compared justification to a snow-covered dung-heap. None have to date (and this author's knowledge) been able to document precisely who originated the phrase, or where it occurs in Luther's voluminous writings, although at least one guy has given it a shot.
Considering that we recently demonstrated that another famous "Luther" quotation was actually a Cochlaean gloss and not Luther's words, I thought I'd try my hand at tracking down where the colorful phrase "snow-covered dung-heap" originated.
Interestingly, the earliest reference I could find was actually rather earlier than Luther himself. William Langland's Piers the Plowman (supposedly a 14th century composition), at Passus 17, line 265, has the expression in late-middle English, which I've updated to modern spellings:
Hypocrisy is a branch of pride - and most among clerics
And is likened in Latin - to a loathly dungheap
That were be-snowed all with snow - and snakes within
Or to a wall white-limed - and were black within
The author of the work goes on to complain of the hypocrisy of priests and to quote John Chrysostom.
One interesting fact about the work is that it came to be printed around the time of Edward VI in England. Because of some major textual variant issues, it's not really easy to say whether Langland actually wrote the version that includes the passage above.
In any event, the passage itself clearly points to some even earlier saying in Latin. Perhaps it is possible that if we dig far enough back we'll find the root source in a patristic writer such as Chrysostom or pseudo-Chrysostom. And perhaps it is just a coincidence. There is, of course, no connection to Luther (which would be chronologically impossible) and the application of the simile is to hypocrisy rather than justification.
Considering the publication date of this work, it is possible that someone picked up the turn of phrase and then considered trying to apply to a view of imputed righteousness. Unfortunately, for now, the mystery continues.
There's at least one other citation that pre-dates Luther, as well. We can see remarkable similarity between the passage above from Langford, and an extract from a sermon ascribed to the German priest Johann Gieler (1445-1510) (without citation):
"In another sermon on existing conditions, he said:
O Lord my God, how falsely now do even those live who seem most spiritual — Parsons and Monks, Beguines and Penitents. Their study is not to work God's works but to conceal the devil's works. Among these all is outward show, and there is no truth, nought else but dung besnowed or buried under snow; without is the glistering whiteness of righteousness and honesty, but within a conscience reeking with vermin and with the stench of sin. The day shall come when the Sun of Righteousness shall melt the snow, and then shall the secrets of your hearts be revealed. And would that the filth of our sins were at least covered with the appearance of snow, that our sin, like Sodom, were not published abroad without shame!"
(link to source) (emphases added)
What's particularly interesting about the quotation is that it is presented on the page I've linked to, just above a quotation from Luther - which might lead the careless reader, flipping open to that page, to think it was Luther's words.
From another perspective, the connection of the quotation to Chrysostom has been linked this way:
The source below, citing "Michael's Ayenbite of Inwyt (Again-bite, or Remorse of Conscience)," which had been edited for the Early English Text Society (though no precise date is given:
"Fairhood is but a white sack full of dung, - stinking, and as a muck-heap be-snewed."
The source says that this "elegant comparison" is an obvious plagiarism from a passage in Chrysostom, in which Chrysostom provides a rather detailed way to rid oneself of lust for beautiful women by mentally stripping them of their skin-deep beauty.
The source itself does not provide a citation to the relevant passage of Chrysostom, but another source (link) identifies it as Homily 7, Section 7, from Chrysostom's homilies on Paul's Second Epistle to the Corinthians. I should point out that most of the popular English versions of that homily do not seem to contain the text in quoted in the source above, or contain it in greatly summarized and expurgated form. A Greek/Latin diglot version provides the relevant passage (here).
Where else can it be found - probably lots of places. After the items above, the earliest I could find the item was in a work by John Berridge (d. 1793) in a sermon of his.
I suppose the matter may remain something of a mystery, unless/until we uncover the first source to associate Luther with the quotation.