The word ‘Protestant’ … originally related to a specific occasion, in 1529, when at the Holy Roman Empire’s Diet (imperial assembly) held in the city of Speyer, the group of princes and cities who supported the programmes of reformation promoted by Martin Luther and Huldrych Zwingli found themselves in a voting minority: to keep their solidarity, they issued a ‘Protestatio’, affirming the reforming beliefs that they shared. The label ‘Protestant’ thereafter was part of German or imperial politics for decades, and did not have a wider reference than that…. It is therefore problematic to use ‘Protestant’ as a simple description for sympathizers with reform in the first half of the sixteenth century, and the reader will find that often in this book I use a different word, ‘evangelical’. That word has the advantage that it was widely used and recognized at the time, and it also encapsulates what was most important to this collection of activists: the good news of the Gospel, in Latinized Greek, the evangelium.
— Diarmaid MacCulloch, The Reformation: A History