Would millions have died if the old religion gave way to the new one without a fight? The problem for the Vatican was that their rhetoric wasn't at top form after mentally stagnating for a few centuries since arguing with Roman pagans, so war was the only possibility to win.
(Don't forget Luther's post hoc justification of killing 100k+ peasants, but he won because he had better rhetorical skills AND the backing of aristocrats and armies. https://en.wikipedia.org/wiki/Against_the_Murderous,_Thievin... and https://en.wikipedia.org/wiki/German_Peasants%27_War)