/u/JimeDorje,

To say that Charles V was "arch-Catholic" in 1527 would be incorrect for the context that is the 1527 Sack of Rome. There were already serious disagreements by that point in 1525-27, but there was not yet large-scale violence between Catholics and Lutherans, although the former considered the latter to be heretics. Certainly, Charles V himself seemed to have preferred accommodation toward Lutherans as he faced great challenges everywhere. Charles held multiple titles, and with those came great conflicting responsibilities. He had to appease the conservatives in Spain, the fractured Holy Roman Empire princes facing Ottoman threats, and himself facing continuing challenge from Francis I of France and the supportive attitude of Pope Clement VII toward Francis.

In fact, with regards to the fate of Latin Christianity, the 1520s saw Charles at his most diplomatic self, showing a far softer side after gnashing his teeth in the 1521 Diet of Worms. The biggest telltale sign being that in 1526 he allowed the Diet of Speyer to provide a poorly-defined armistice between Catholics and Lutherans, as he himself failed to attend the Diet and sent his brother Ferdinand instead. Charles' decision was informed by his falling out with Pope Clement VII, who had released Francis I of France from the harsh settlement he was forced into after being captured in Pavia; and by Charles' ongoing inability to corral German princes -- heretics or otherwise -- to rally behind him in his ongoing wars against the Ottomans. He was then forced to negotiate with German princes with little support from the Pope. He had instructed his negotiators in the Diet of Speyer to demand the implementation of the imperial ban given in the 1521 Diet of Worms, but when the princes pushed back against his regent Ferdinand, he didn't forbid them from coming to significant accommodation.

The earliest point of no return in terms of the split between Catholics and Lutherans is usually given as the 1529 Diet of Speyer, where Charles once again demanded that all princes obey the Imperial ban of the 1521 Edict of Worms. Further, due to Ottoman advances against Vienne, he asked them to contribute money and troops into his imperial wars against the Ottomans -- else face consequences. Once again, Charles was absent due to wars elsewhere, and was represented by his brother Ferdinand once again. This time, Ferdinand read Charles' message more strongly, and worked to bring as many Catholic princes as he could. Here the Lutherans, Zwinglians, Calvinists, et al., felt they were being outnumbered and delivered a Protestation. Thus, they came to be called Protestants. The written Protestation ended up not being read at the meeting, but it was widely published and circulated. It opened up a political rift that many see as having signaled a point of no return even if the following year's Diet of Augsburg once again pondered armistice between Catholics and Lutherans.

In summary, the 1525-1529 period saw Charles at his most accommodating self toward heretics, and did not yet strongly draw a line in the sand that later led him to the Schmalkaldic War, the first open warfare between Catholic and Protestant powers. Notably, both Charles V and Martin Luther were present only at the 1521 Diet of Worms. In all other imperial Diets, both were absent: Martin Luther because he was an outlaw and feared for his life, Charles V because he had (what he thought to be) more pressing commitments elsewhere. Even as Melanchthon and Valdes attempted reconciliation, the winds of change blew against them.

So, I believe you are taking a good approach in reading the history of Reformation alongside the ongoing developments in politics and warfare. In case you were interested in aspects of the political development leading to 1527, you may want to read what happened ahead of the 1525 Battle of Pavia.