Protestant comes from the term protest. Back in the 1500's, it was formed to protest a decree set by the Holy Roman Empire. For a more complete description and history, look at http://www.newadvent.org/cathen/12495a.htm

The term Protestant refers to those that protest against the abuses of the the Roman church. "Protest" in late Middle English originally meant to make a solemn declaration. It came to late Middle English from the Old French "protester" and the Latin "protestari." This is a compond word of the Latin "pro" and the Latin "testari." Pro is "publically," while "testari" means "assert." The term Protestant comes from the Evangelical Lutheran response to a clause in the reactionary papistic Diet of Spires that reinstated the Edict of Worms. The Edict of Worms was replaced with the Edict of Spires, but the papistic party wanted to reinstate it. The Edict of Worms declared that all of Luther's books be seized and burned. Luther was declared an outlaw, to be killed by anyone without fear of punishment. It said "we forbid anyone, regardless of his authority or privilege, to dare to buy, sell, keep, read, write, or have somebody write, print or have printed, or affirm or defend the books, writings, or opinions of the said Martin Luther" (translation by De Lamar Jensen and Jacquelin Delbrouwire). King Henry the Eight killed all the Lutherans in England and recieved the title "Defender of the Faith" from the pope. The Diet of Spires declared that the current Lutherans would be allowed to live, but it further conversions were prohibited. It also essentially meant the usurpation of the local German princes' right of governing their own state without oppression by the Emperor, who under Imperial Law had no right to wage war against the princes. The Emperor Charles V couldn't come, so he had his brother, King Ferdinand, come as a substitute. Ferdinand said there was nothing for the Lutherans to do but submit, and then he went home. But the Lutherans came back in the morning to read their solemn declaration before those present and the King's empty chair. The "we protest" in the document the Lutherans read was a turning point in the history of liberty and freedom in Europe and the World. Evangelical princes formed the Smalcaldic League to protect themselves from the papists that didn't want them to have religious freedom.