The following commentary is from the EWTN series, The Papacy: A Living History, The Papal Artifacts Collection of Father Richard Kunst. This papal bull from the only anti-pope in the Collection was featured on Episode 5 of the series, Popes of the 17th Century and Earlier.
Here is Father’s Commentary:
The lead seal that we have here is a papal bull that was originally attached to a parchment, and it fell off, but this is such a unique papal bull. I have several of them in the Collection, but this is the only item I own that is from an anti-pope.
All throughout church history we have had people who have claimed to be pope. An anti-pope is a person who claims to be pope and who may have significant followers, but is not the legitimate Bishop of Rome.
Some of them have had strong arguments that suggest they are the legitimate popes, and we hear about a period of time in the 1300s and into the 1400s where we had the “great schism.” That was when two men claimed to be popes at the same time, or even three claimed to be popes at one time. So we have had men that have claimed to be pope at one time or another for different reasons, and some of them had strong arguments. And we hear in that period of time where various groups of cardinals went off and voted for pope or the other.
This is actually a papal bull from the anti-pope John XXIII, who was in the first part of the 1400s. And because he was an anti-pope, the name was still open and available for another to use. There have been some situations in papal history that when an anti-pope uses a name, the name is never used again. In other cases they might choose to go to the next number. As a matter of fact, this is true in the use of the name, John XXIII. The mere fact that Blessed John XXIII was elected in 1958, when the last John XXIII died in the 1400s shows the amount of time that elapsed before the name was used again.
So it took 500 years for someone to feel comfortable to use that name again, and it’s the most used papal name: there are twenty-three Pope Johns, and sixteen Gregories and sixteen Benedicts.
So it’s a great item to look at Pope John XXIII but to know, in this case, that he’s an anti-pope.
The word, bull comes from the Latin, bubble. It is the lead seal that was appended to the end of a document acting as the signature of the pope. On one side, it contains the pope’s name and on the other, images of Saints Peter and Paul from whom he is given his authority.
The bull is generally an official document of the Holy Father. We know by the choice of threading used, either silk or twine, the importance of any particular bull.
There is more information on anti-popes on Papal History/Ask the Curator.