The Meaning of "a cappella"

 Most of us (including me) have been educated to believe that "a capella" means "singing without instrumental accompaniment". But Michael G. Knox recently educated me further. He writes (with minor edits):

I’m glad you know what “a cappella” means -- in the modern context. But that wasn't originally the case.

The very reason that term was invented was to differentiate one style of orchestration from another. It did not originally mean “singing without instruments.”

One of the earliest uses of the earlier version “alla capella” (Italian, meaning “according to the chapel”) was in 1742 by the German composer Johann Galliard. He used the term in a secular composition to denote that the style of the song was to mimic “chapel” music of the day. Since, in his experience, the chapel music of the mid-1700’s (Catholic) would have been instrumental, he was referring to a style of accompanied singing. He intended for the music to play in unison with the singers instead of being complementary of the singers. Over time, the term “alla capella” took on the Latin spelling “a cappella” in 1868 when it denoted "that instruments are to play in unison with the voices, or that one part is to be played by a number instruments." ["Chambers's Encyclopaedia," 1868]. In 1875, "a cappella" referenced, for one of the first times, non-instrumental music. But, as other chorales, sung as part of the church service, were written in the same and simple style, the expression "a capella" came in time to be applied to them also, despite their being sung without any instrumental accompaniment whatever. [The Music World, Sept. 11, 1875]

In time, "a cappella" came to mean singing without instruments. How did this happen? It seems this linguistic shift came about because of the rise of Protestantism in the 1800’s. Most Protestant churches did not use instruments with their singing. Several prominent leaders, including John Calvin vehemently opposed instruments in worship on the grounds it was “too strongly tied to antiquated and unorthodox methods from before the Reformation.” So, in the late 1800’s and early 1900’s the normal “chapel” style was only singing and no instruments. Therefore, when one thought of music in the chapel style ("a cappella"), they thought of solely vocal music.