Here are some key points to understand:
- Before, during, and after the Middle Ages (Black Death, 100 Years' War, Scholasticism, etc.), Popes of the Roman Church and Emperors of several kingdoms maintained a rocky relationship with one another. There are several reasons why, but know that they both sought to gain power at this time, and their roles and responsibilities often intermingled. Sometimes kings sought to appoint religious officials (lay investiture), which bothered the Church; sometimes Popes sought to expand their authority over kings.
- These back-and-forth disagreements led to the "ordination" of a number of popes, excommunications, and much more.
- These disagreements help to explain why the Holy Roman Empire was a conglomeration of feudal land holdings (led by princes, feudal lords, etc.), and why its power petered off during Frederick I's reign as emperor.