Tolkien did not actually give the essay a title, but Christopher used this title because his father used it in the essay in referring to the rejection of the shibboleth of Fëanor. Christopher wrote about this essay:
The essay revolves around the change of þ to s in Noldorin Quenya, and the events with which this alteration became 'entwined'. This change apparently occurred before the darkening of Valinor and the Noldor’s exile, but after the separation of the Noldor and Vanyar (when the Vanyar went to live at the foot of Taniquetil). It apparently was a conscious and deliberate change agreed to by a majority of the Noldor, although both the Vanyar and the Teleri continued to use þ, which took place after the birth of Míriel but before the birth of.In all my father’s last writings linguistic history was closely intertwined with the history of persons and of peoples, and much that he recounted can be seen to have arisen in the search for explanations of linguistic facts or anomalies. The most remarkable example of this is the following essay, arising from his consideration of a problem of historical phonology, which records how the difference in pronunciation of a single consonantal element in Quenya played a significant part in the strife of the Noldorin princes.
This change was attacked by the linguistic loremasters, the chief of whom was Fëanor. "He insisted that þ was the true pronunciation for all who cared for or fully understood their language."
But Fëanor also had a personal reason for attacking this change. The “mother-name” of his mother Míriel was þerinde (needlewoman) because of her skill in embroidery. Míriel, though of gentle was spirit, had a deep streak of obstinacy (as later would be seen), that she bequeathed to her son. She insisted that all of her kin continue to use þ, at least when pronouncing her mother-name. And while she lived, Finwë himself continued to adhere to that pronunciation.
However, things changed after Míriel’s death, and Finwë’s remarriage to Indis. As a Vanyar, Indis might have been expected to be an ally of Fëanor’s in this point. But instead, she adopted the Noldorin use, which had become almost universal among the Noldor after Míriel’s death, particularly since Finwë himself accepted the change after Míriel rejected his pleas to return to him.
To Fëanor, the rejection of þ became a symbol of the rejection of Míriel and of himself, as the chief of the Noldor next to Finwë. Fëanor was the 'son of þerinde' and told his sons to take no heed of the use of s for þ in the house of Finwë and elsewhere. 'We speak as is right, and as King Finwë himself did before he was led astray.' Thus we see reflected in this minor linguistic distinction the roots of the whole dispute that led to the strife among the Noldor.
The use of the word shibboleth by Tolkien here is particularly interesting. Modernly, the word shibboleth is defined as “a word or pronunciation that distinguishes people of one group or class from those of another.” However, it has biblical roots. Shibboleth is the Hebrew word that literally means "torrent of water" or "stream". In the Hebrew Bible, pronunciation of this word was used to distinguish members of a group whose dialect lacked a "sh" sound from members of a group whose dialect included such a sound. The consequences of getting it wrong were fatal:
I find this intersection between Tolkien’s linguistic musings, his own legendarium, and biblical lore incredibly fascinating. This idea of linguistic differences being such a stark reflection of the differences that erupt between humans is one that I have not really considered before. And I can think of few other instances of such a direct biblical reference elsewhere in Tolkien's work. I’m going to have to give more thought to this. And I look forward to any thoughts that anyone else might have about the matter.And the Gileadites seized the passages of the Jordan before the Ephraimites; and it was so, that when those Ephraimites who had escaped said, "Let me go over," that the men of Gilead said unto him, "Art thou an Ephraimite?" If he said, "Nay," then said they unto him, "Say now 'Shibboleth.'" And he said "Sibboleth," for he could not frame to pronounce it right. Then they took him and slew him at the passages of the Jordan; and there fell at that time of the Ephraimites forty and two thousand. (Judges 12:5-6, King James Version of the Bible)
I should add that Tolkien also has some very interesting things to say about Galadriel in this essay (some of which was actually printed in the Unfinished Tales section about Galadriel and Celeborn). But those thoughts will need to wait for another day (and perhaps another thread).