Which Christian hymns were originally drinking songs?

Well, one that does NOT fall into this category is Luther's "A Mighty Fortress Is Our God," which is often claimed to have that origin.

QUESTION: Did Luther use a drinking song as the basis for "A Mighty Fortress Is Our God"?

This is a common misconception, but the answer is an undeniable "no." Martin Luther wrote both the words and the tune for "A Mighty Fortress Is Our God" (in German "Ein feste Burg ist unser Gott").

https://en.wikipedia.org/wiki/A_Mighty_Fortress_Is_Our_God

This site also insists "No" for any of Luthers' or Wesley's hymns.

http://www.gbod.org/worship/default.asp?act=reader&item_id=2639&loc_id=17,387

Although there is a popular Christian hymn song called "One Hundred Jars of Clay On The Wall" which is sung to the tune of "100 Bottles of Beer On The Wall".

http://www.youtube.com/watch?v=o5DJXa0n05c

Qp's just disabusing herself of this popular misconception, not specifically about "A Mighty Fortress," but the notion that classic Protestant hymns sprang from the popular culture.

The Lutheran Church identifies four sources of Luther's tunes, and mentions only one as having come from a tavern tune. The music was changed.

"The first stanza of "From Heav’n Above" (ELW 268) was based upon a light-hearted love song, "From Distant Land I Come To You." Originally the hymn text was sung to the love song tune. Since this tune was still being sung in taverns and other places for entertainment, it was not long before the reformers substituted a newly composed melody for the original."

http://download.elca.org/ELCA%20Resource%20Repository/Did_Martin_Luther_really_use%20tavern_tunes_in_church.pdf

The opposite holds in the case of Gaudeamus Igitur, which because of its Latin words is sometimes mistaken for a hymn. That is based on an earlier hymn tune, and is a student drinking song.

https://en.wikipedia.org/wiki/Gaudeamus_igitur

See also Brahms's Academic Festival Overture.

Can I suggest someone looks into the Sally Army repertoire? ('Why should the Devil have all the best tunes?')

Not a drinking song as such but quite possibly a song about a lady of 'negotiable affection', Greensleeves has been used as a hymn tune - for What Child is This.

Also, in the medieval and post medieval times, there wasn't the distinction between sacred and secular in music that we have now. A tune that did for a ballad also did for a hymn. The droned dirges about joy that characterise Anglican services for me are a relatively modern phenomenon. For real joy, try Hildegard of Bingen

https://en.wikipedia.org/wiki/Hildegard_of_Bingen

The Anglicans might celebrate her as a saint, but they ignore her musical lessons...)

Baloo, this question came from a correspondence with player Cymruambyth about the Devil's tunes in her excellent new quiz, "The Devil You Say!" Probably not allowed to advertise a quiz here.
http://www.funtrivia.com/trivia-quiz/Humanities/The-Devil-You-Say-309518.html

But this should be allowed:

"For a time, the composer and arranger Benjamin Jacob was organist, attracting thousands; a practical response to Rowland Hill's well known concern about chapel music of the time: 'Why should the Devil have all the good tunes?'."
https://en.wikipedia.org/wiki/Surrey_Chapel

See also http://www.av1611.org/question/cqdevila.html