I have been carrying the wrong date around in my head for at least a decade. Felix Manz, evangelist, thinker, and martyr, was drowned in the Limmat on January 5th, 1527, not January 21st, 1525. The earlier date was when he began his public ministry. So have a few minutes on the internet educated me.
Before the internet, the last big advance in technology for people to communicate their thoughts to other people was movable type. This had all sorts of profound social and political consequences which you can look up for yourselves on the internet.
Basically, if you wanted to communicate your ideas to posterity, before movable type you either needed really compelling ideas that made lots of other people want to copy them out longhand, or ideas backed up with enough money and/or power that you could compel lots of other people to copy them out longhand. This was a high bar to get over, so the quality of the ideas communicated to us from the time before the invention of movable type is generally high.
Once movable type was invented, all you needed was a relatively trivial amount of money, and you could communicate whatever thoughts popped into your head to every town that spoke your language within weeks or months. If you disagreed with something someone else wrote, you could dash off a pamphlet letting the world know, and a few weeks or months later they would do the same, usw. The content of the resulting deluge of pamphlets will be reassuringly familiar to anyone browsing the net today. The hyperbole, the rapid-fire cut and thrust of responses, the bees in bonnets, the self-righteous separation into self-reinforcing cliques- it is all so very familiar.
From the 1911 Encyclopedia Brittanica:
"In Germany, the cradle of printing, the pamphlet (Flugschrift) was soon a recognized and popular vehicle of thought, and the fierce religious controversies of the Reformation period afforded a unique opportunity for its use. ... In general their tone was extremely intemperate, and they formed, as one authority has described those of a century later, "a mass of panegyric, admonition, invective, controversy and scurrility." ... attempts were made in pamphlets to justify almost every action, however unjust or dishonourable, while at the same time those who held different opinions were mercilessly and scurrilously attacked."
I don't know if Felix Manz was the first Protestant killed by other Protestants for having the wrong ideas, but he was early enough- January 5th 1527 is less than ten years after the 'beast of the wild wood' (to use Gerard Manley Hopkins delightfully un-ecumenical phrase) nailed his theses to the church door in Wittenberg. That's what happens when people get carried away with these new technologies for communicating their ideas. He has stuck in my mind since I first heard about him because of how shockingly early in the Reformation he was judicially murdered, and because some of my ancestors almost certainly had a hand in his death, as the branch of our family that has been traced furthest back has been traced to 16th century Zurich.
Here is a sample of Felix Manz's dangerously inflammatory opinions:
"Charity alone is pleasing to God; he that cannot show charity, has no part with God. The unadulterated love of Christ puts to flight the enemy. It is incumbent upon him that will be an heir of Christ, to be merciful, as the Father in heaven is merciful."