There have been MANY anti-Christs (and a few have also been Popes).
It’s difficult to tell if Revelation’s anti-Christ is a particular individual or a composition of many anti-Christs.
Martin Luther was the first person to associate the Pope with the anti-Christ. Although the Pope during his time (Leo-10) was not one of the “bad” Popes. He was an ineffective administrator (some would say disastrously ineffective, which would be difficult to argue), but he was not personally wicked.
If there is such a thing as THE anti-Christ, and if he will show up at the end, then obviously Leo-10 could not have been THE anti-Christ (heck, he wasn’t even AN anti-Christ). As anti-Catholic rhetoric continued to develop there arose this idea that SOME Pope would be THE anti-Christ. Nobody knows which one, but since there is a common mindset among many protestants that we’re always in the “end times” then they figure whomever is Pope today must be THE anti-Christ.
Although how anybody could look at Pope Francis and imagine such a thing is beyond my comprehension.
Trivia: Pope Leo-10 was the most recent Pope who did not hold priestly Orders at the time of his election.