Final answer:
Pope St. Gregory deposed Henry IV by excommunicating him and nullifying his rule, which instigated a rebellion by German princes, undermining Henry’s authority.
Step-by-step explanation:
Pope St. Gregory was able to depose Henry IV through a combination of asserting spiritual authority and manipulating political alliances. Despite having no military power, popes like Gregory VII wielded influence by creating relationships with kings and overseeing Christian missionary work. When Henry IV continued the practice of simony, defying Gregory's ban, the pope excommunicated him and declared his rule illegitimate. This led to a rebellion against Henry IV by German princes, leveraging the view that without the pope's spiritual authority, Henry's power was meaningless.