Why did Pope Gregory the Great say the “universal bishop” is the Antichrist?

How? By carefully observing how the term is used differently in different instances. In its approved sense, the title “universal bishop” suggests that the Bishop of Rome’s jurisdiction and authority extend to the whole Church, something with which Gregory was in hearty agreement.

But it can be used in an incorrect sense also, and it is this sense that Gregory condemned. In the condemned sense the title is taken to mean that in the Church there is only one true bishop, with all others who claim the title merely acting as the true bishop’s delegates or deputies.

Although Gregory believed the papacy to possess a universal jurisdiction and supremacy of authority, he didn’t think, nor does the Catholic Church teach, that this means only the Bishop of Rome is truly a bishop endowed by Christ with the power to teach, sanctify, and govern in Christ’s name.