When were Protestants first called 'Protestants' and what were they, originally, protesting about?

Question

I am asking, precisely, the first time when 'Protestants' were so called, either among themselves or by others. And what, precisely, they were in protestation regarding.

There is a previous question about the 'Protestant Precursors' but that is not my question here.

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My question is prompted by the 'disclaimer' to a previous question

Disclaimer: I understand that there is no monolithic Protestant belief on any topic except a protestation of the Catholic Church.

I would have thought that it was historically evident that the doctrine rising high above all others in the Reformation was Justification by Faith and I am interested to see if that is substantiated by the facts.

Namely, that the 'protest' was not a negative, reactionary matter but a positive matter asserting a doctrine (justification by faith, regarding, specifically, the righteousness of God) to be received by a genuine faith.

Answer 1

The year was 1529, the event was the Second Diet of Speyer, and the particular matter that brought about labeling of Evangelicals as 'Protestants' was their objecting to undemocratic religious prejudice regarding the reaffirmation of the Edict of Worms.

They did not give themselves that name but the label stuck, yet became much misunderstood by those unacquainted with the circumstances as to what, precisely, the Evangelicals were protesting about:

The Edict of Worms was reaffirmed only for Catholic territories. Provisionally, and prior to reaffirmation of the Edict of Worms by the general council, Lutheranism was to be tolerated in those regions where it could not be suppressed without tumult. By then, most of northern Germany had become Lutheran, and in the south the cities of Strassburg, Augsburg, Ulm, and Nurnberg. Constance embraced the reform, severed connections with the Hapsburgs, and joined the Swiss. Basel came over to the reform in 1529, the year of the Second Diet of Speyer. Between June 25th to August 27th 1526, the Diet of Speyer deferred action on the Edict of Worms. Here I quote:

"In Lutheran lands, the principle of religious liberty for Catholics must be observed, whereas in Catholic lands the same liberty would not be extended to the Lutherans. Against this invidious arrangement the Evangelicals protested, whence the origin of the name Protestants. They contended that the majority of one diet could not rescind the unanimous action of the previous assembly. They questioned whether this was the intent of the emperor, and on that score they were correct. They affirmed that they could not have two religions side by side in their territories without menace to the public peace, and if their plea was not heard, then 'they must protest and testify publicly before God that they could consent to nothing contrary to his Word.'

Their stand has been variously misinterpreted. In the Protestant camp the emphasis has been all too much on the first word, 'protest', rather than on the second, 'testify'. Above all else, they were confessing their faith... In this protest the Zwinglians and Lutherans were joined." Here I Stand by Roland Bainton, pp 317-8 (Lion, 1988)

I have gone over and above the simple question in order to put a little flesh on the bare bones of the answer: "April 19th, 1529".

Answer 2

To attempt to add clarification about the Protestant name and who gave it to whom, this is from here.

The "Letter of Protestation" was signed by Johann, Elector of Saxony, Georg, Margrave of Brandenburg, Ernst, Duke of Braunschweig-Lüneburg, Philipp, Landgrave of Hesse, and Wolfgang, Prince of Anhalt.

So, the term Protestant would have erupted from the Letter of Protestation.

They protested against the Catholic Church's ban on Luther, his works, and called for the spread of evangelical faith (priesthood of believer, etc). Here is a translation.