The "Pro-Catholic" group includes followers of the Catholic church, where the "Pro-Protestant" group includes those who opposed the territorial rule that the Catholic church held. Because it was in such a high seat of power, the Church owned parts of each territory of Europe based on the strength of their economy and also collected an array of different taxes. Germany in particular was the weakest politically and economically, so they were hit hardest by the Church’s rule. This unbalanced way of taxation was the cause of the reformation, as the people could not support the needs of the Pope; therefore, they used religion as a rallying point.
This caused leaders like Martin Luther to rise to power and with the invention of the printing press, allowed his copies of his 95 thesis to spread throughout Europe. Because Luther was so well known and the emperor’s relations with Germany were already sour, it was not in his interest to execute Luther when he was summoned to Rome. Although tensions were still high and the followers of the new religion were not left alone until some time after, it still managed to spread across Europe because of the rights it had to offer.