Charles wasn't in the room. He'd spent the better part of thirty years trying to hold one church together in the empire, and when the settlement was finally hammered out at the diet in 1555 he stayed away and let his brother Ferdinand preside. You can read that as exhaustion or as conscience or as both; the sources don't agree, and I don't think Charles ever quite said. He abdicated not long after.
So it was Ferdinand, and the princes' representatives, and clerks — men whose names mostly didn't survive — going line by line over what a territory's religion actually meant in practice. Lutheran or Catholic: those were the two doors. Calvinists weren't in the text at all and wouldn't be for nearly a century. And there's a clause I always stop at, the one about leaving: if your prince's confession wasn't yours, you were allowed to sell up, take your family and go somewhere it was, without forfeiting everything. A right of emigration rather than a right of conscience.
Which sounds thin, and was thin, and was also more than most people in Europe had at the time. Somebody had to pack the cooking pots. Somebody had to decide whether the vineyard was worth more than the sacrament, and there's no ledger of how that went, household by household — only the clause, sitting in the treaty, doing its quiet work for the next sixty-odd years.
Ferdinand signed as his brother's deputy, and the record shows he'd been doing the empire's negotiating for years by then — Charles handed over the imperial crown within months. A settlement concluded by a man acting for someone who couldn't bear to be there.
The tag everybody quotes at it, cuius regio eius religio, isn't in the document at all — a Greifswald lawyer, Joachim Stephani, put it that way decades later, and it stuck because it scans. Whose realm, his religion; four words doing the work of a treaty.