I'm not sure if the emphasis is to be put on the fact that the <em>Pope</em> declared or that the event occurred. I'll answer both.
It was significant that the Pope declared it because he was pretty much the only person who had the ability to declare such a thing. Even though the kingdoms of Europe had different rulers, most kingdoms were based on the Catholic religion. Therefore, the Pope had influence. If he declared something, it was as good as God declaring it.
It was significant that a Holy Roman Emperor was declared because it was the first step -- of many -- to achieving German unification. Before Charlemagne, Germany was nothing more than the Germanic tribes. The Holy Roman Empire, which lasted until 1806, kept these kingdoms united.
I would say that King probably said that people have a moral right to obey just laws just as they have that same moral right to disobey unjust laws such as the laws of segregation between whites and black Americans in some southern states.