The Thirty Years' War in Central Europe began when the Anglican Church was established in Germany and was met with resistance. Lutheran princes challenged the Holy Roman Emperor, who was Catholic. Lutheran princes battled Huguenot princes for control of German states. Calvinists went to Germany to spread their beliefs and establish strict laws.

I believe the answer is: Lutheran princes challenged the Holy Roman Emperor, who was Catholic.

After the war, both sides agreed that continuing the conflict would only resulted in unecessary death on both sides.

Because of this, The Peace treaty of Ausburg was created. This treaty make both catholic and lutheran agree that All Princes are guaranteed the right to choose either lutheran or catholic as their religion based on their own belief.

Since it was a war of Protestants and Catholics, the most correct answer is

Lutheran princes challenged the Holy Roman Emperor, who was Catholic. That's how it began but it involved many other states as it developed.