From the Biblical perspective, nationalism is a beautiful thing
In order to make arguments for nationalism, we have to define it.
The first definition in Merriam-Webster is "loyalty and devotion to a nation." But in a second paragraph, it adds, "especially: a sense of national consciousness exalting one nation above all others and placing primary emphasis on promotion of its culture and interests as opposed to those of other nations or supranational groups."
Let's be clear: If the second paragraph is the only definition of nationalism, nationalism is always a bad thing. Furthermore, I acknowledge that this definition is what some people have in mind when they call themselves nationalists.
At the same time, even anti-nationalists would have to acknowledge that if the first paragraph is the definition of "nationalism," nationalism can often be a beautiful thing.
So, if we are to be honest, the answer to the question of whether nationalism is good or bad is "How do you define it?"
Dictionary.com offers seven definitions.
The first is "spirit or aspirations common to the whole nation."
The second is "devotion and loyalty to one's own country; patriotism."
Only when we get to the third definition is the definition pejorative: "excessive patriotism; chauvinism."
Therefore, a) based on the competing definitions of the term, b) assuming both definitions can be true and c) if intellectual honesty is to govern our discussion, we can reach only one conclusion: There is good nationalism and bad nationalism.