How can the Protestant Reformation be considered a nationalist movement?
The Protestant Reformation can be viewed in some ways as similar to a nationalist movement. The Protestant Reformation was a movement to reform some of the practices of the Catholic Church. An example of reform stemmed from the displeasure that people had with the Catholic Church selling indulgences. Martin Luther felt this was not appropriate. Martin Luther was eventually excommunicated from the Catholic Church.
Various people in different countries revolted against the Catholic Church and its practices. For example, Martin Luther supported the people in Germany who revolted. This was similar to people who wanted to be free from a colonial power, except, in this instance, a new religion was developing, namely, the Lutheran religion. The Lutheran religion spread to many places including Germany, the Scandinavian countries, and the Baltic regions. Because a new religion was forming, this was similar to a nationalist movement.
Part of the reason why Henry VIII, the King of England, broke away from the Catholic Church was so that his decisions did not have to be agreed to by the Pope. Henry wanted a divorce from Catherine of Aragon so that he could produce a male heir, but the Pope would not agree to annul this marriage so that Henry could marry Anne Boleyn. Though Henry was, earlier in his life, a devout Catholic, he resented the power that the Pope in Rome had over his decisions. In splitting from the Pope in the Protestant Reformation and declaring himself the leader of the Church of England, he established independence for the nation of England and for his new religion. Therefore, breaking away from Catholicism was a form of nationalism. In later years, Protestant England became a rival of Catholic Spain, as nationalism was partly based on ethnic identity and partly on religious identity.
The legacy of the Reformation is still evident in Europe today, as the split between Catholicism and Protestantism runs largely along regional and national lines. In Italy, there are virtually no religions besides Catholicism that have a major following. In Denmark, it is universally Lutheran - so much so that no one there refers to it as the "Lutheran Church". It's just "Church" because there are virtually no other ones to differentiate from.
The Protestant Reformation planted its deepest roots in Northern Europe and Scandinavia, and it's still true today. It represents a long lasting schism between Catholic and Protestant that's permanently affected the human geography.
I see a couple of ways that you can argue this, even though I am not at all sure that I agree with them.
First of all, Germany was split into many little kingdoms during those days. You could say that each of these was a nation that could be nationalist. Martin Luther was supported by various rulers of these little kingdoms. They could use the new religion as a way of creating solidarity in their nations against others.
Second, the Catholic Church is run from Rome. You can argue that it is nationalist to have Germans push for a religion that they themselves control. Since Lutherans did not obey any outside authority, you can say they were nationalist.