What was the most important battle for the Protestants and Catholics during the 16th century when the Protestants were becoming a religion?

Answer 1

The two most important "religious" wars of that era were the Eighty Year's War between the protestant Dutch Republic and catholic Spain; and the Thirty Year's War between the protestant and catholic States in Germany. Both wars drew in other countries as supporters and participants; both wars ended with the Peace of Westphalia in 1648.

The eighty Year's war was the only one that already formally started in the 16th century, but the Thirty Year's war - although started in the early 17th century - was the result of tensions and fights that had started in the 16th century. Both wars did not have some singe battle that determined the outcome; it was the result of many ongoing campaigns - and people wanting to see the end of it - that finally made all the participants want to sue for peace.