Today marks the anniversary of the birth of the term “Protestant,” but its origin is frequently unknown or ignored. The term does not come from the camp of theologians, but from that of magistrates acting as the heads of their realms. It is a term, in other words, that reminds us of the importance of the term magisterial in the common phrase “Magisterial Reformation.”
The term comes from the second Diet of Speyer in 1529 (the first had been in 1526). Here is a brief précis of what transpired at each from the Oxford Dictionary of the Christian Church:
Speyer (Spires), Diets of (1526 and 1529). (1) The Diet of June 1526 marked a new stage in the consolidation of reforming influences in Germany. Instructions sent by Charles V from Spain in the previous March forbidding all innovations and requiring the enforcement of the Edict of Worms (1521) were set aside on the ground of his war with the Papacy which had broken out in the interval. The Diet determined that each Prince should order ecclesiastical affairs in his state in acc. with his own conscience.
(2) By the end of 1528 a Catholic reaction had set in, provoked by Philip of Hesse’s repressive measures and his invocation of foreign (French and Hungarian) aid. In response to an appeal from the Pope, Charles V issued a mandate on 30 Nov. 1528 summoning the Diet to Speyer on 21 Feb. 1529. The proceedings were controlled by a strong and well-organized Catholic majority who passed legislation to end all toleration of Lutherans in Catholic districts. On 19 April five Princes and 14 cities made a formal ‘protest’ addressed to the Archduke Ferdinand, defending freedom of conscience and the right of minorities. Henceforward the Reformers were known as ‘Protestants’.
“Protestantism” did not come from Jeffersonians, then.
We can gather a bit of additional background from Heiko Oberman’s Luther:
Philip does not seem to have chosen the side of the Reformation for selfish or political motives. But shortly after his inner conversion, he began drawing political consequences from his profession of faith. First he strove to make a defensive pact with Electoral Saxony. His wish was realized in 1526 with the Gotha-Torgau treaty, but he could consider this treaty no more than the beginning of a larger pact system that would place the Evangelical estates in a position to hold their ground against pope and emperor. He wanted all the Evangelical territories and cities to unite. His ambitious plans were, however, foiled by the eucharistic controversy between Luther and Zwingli, which took on increasingly violent forms in the years 1525 to 1528. The Hessian landgrave was anxious to draft a treaty that allied northern and southern Germany and also included Evangelical Switzerland. Electoral Saxony, however, insisted that a unified profession of faith must precede the conclusion of any treaty to serve as the basis of the political alliance.
For two years Philip had been trying in vain to win Luther for his plans, when then imperial diet in Speyer (1529) made a move that could not be ignored: the imperial estates, overruling the minority of Evangelical estates, decided it was finally time to wipe out the Reformation, in keeping with the instructions of the Edict of Worms. The Evangelical estates lodged protests against the decision on April 19 and 20, thus gaining themselves the name “Protestants.” The Protestants objected to the diet’s transgression of authority “as in matters of God’s honor and the salvation of our souls, each [estate] must stand for itself before God…”
The “Protest” of 19 April no longer exists, but a more detailed one was put together on the 20th; you can see an English translation here.