It just occurred to me that I have no recollection of any discussion of the oldest acronym (or initialism).
I googled and found on the first page: ICHTHUS and SPQR, this latter being called in the item heading "the oldest acronym in current use", though its current usage might be disputed.
Also,
http://www.curiousnotions.com/home/mene.html , I always pictured this as an acronym (in English language, though, and in my minds' eye.
I was familiar with ICHTHUS, but wondered if INRI in the Christian mode mightn't be older, or if it is something made up later, after the Christians came out from underground.
"The letters "INRI" are initials for the Latin title that Pontius Pilate had written over the head of Jesus Christ on the cross (John 19:19). Latin was the official language of the Roman Empire. The words were "Iesus Nazarenvs Rex Ivdaeorvm." Latin uses "I" instead of the English "J", and "V" instead of "U" (i.e., Jesus Nazarenus Rex Judaeorum). The English translation is "Jesus of Nazareth, the King of the Jews."
The Early Church adopted the first letters of each word of this inscription "INRI" as a symbol. Throughout the centuries INRI has appeared in many paintings of the crucifixion.
By the way, Pilate's title for Christ was actually written in three languages. etc."
http://www.christiananswers.net/q-eden/edn-t024.html