Theologian Fred Sanders clarifies the meaning of “Protestant”:
Today (April 19) is the anniversary of the 1529 Protestation of Speyer, which is generally regarded as the first time that the word “Protestant” was used to refer to a religious position distinct from Roman Catholicism. A coalition of German princes and leaders refused to abide by the imperial ban on Luther’s teachings, and called instead for the free spread of gospel teaching in their territories.
These days, in English at least, we sometimes hear that “Protestants” are by definition people who “protest,” that is, people defined by their disagreement with something, their dissent, their rejection of something. It is, in other words, considered a term that stands for nothing positive, but draws its meaning only by negation.
Now, I don’t make much of this, but it seems to me like a bit of bogus etymology. “Protest” might be the nearest cognate of “Protestant” in modern English, but it’s silly to take that as a clue to the word’s origin –sort of like finding “dance” in the word “concordance” and deciding they’re related; or “sacrilege” means putting religion in a sac; or that “validate” is from valid + date = “at the right time;” or “excruciate” means to take off of a cross, etc. But I digress.
The word seems to come from pro + testari, to testify forth, or to hold forth a position on something. Its primary historical meaning has been to assert, to maintain, to proclaim solemnly or state formally.