Baldassarre Cossa (c. 1370 – November 22, 1419), also known as John XXIII, was pope or antipope during the Western Schism (1410–1415) and is now officially regarded by the Catholic Church as an antipope.
He was one of the seven cardinals who, in May 1408, deserted Pope Gregory XII, and, with those belonging to the obedience of Antipope Benedict XIII, convened the Council of Pisa, of which Cossa became the leader. They elected Pope Alexander V in 1409. Cossa succeeded him a year later.
Edward Gibbon asserts in The History of the Decline and Fall of the Roman Empire that John XXIII was charged with piracy, murder, rape, sodomy, and incest, with the more serious charges being suppressed. Of course, this must be viewed in the light of the political situation of the time, as the charges were likely trumped up; note the similarity to the charges against the Knights Templar.
He should not be confused with Pope John XXIII of the twentieth century. The fact that there were a number of Popes named John during the first 1400 years of the Church and then no more for over 500 years is probably due to the controversial figure this Antipope represented. When Angelo Cardinal Roncalli was elected and became Pope John, there was some confusion as to whether he would be John XXIII or John XXIV; he then declared that he was John XXIII to put this question to rest. The decision of the twentieth century Pope John XXIII not to be named John XXIV as might be expected serves as a confirmation of the antipope status of this first John XXIII. It should be noted, however, that the numbering of the Popes called John is debatable (as there was no John XX); for example, Gibbon refers to the Antipope John as John XXII.
With the aid of the Emperor Sigismund, Pope John convened the Council of Constance in 1415. During the third session rival Pope Gregory XII authorized the council as well, and soon both Popes abdicated in favor of Pope Martin V. Cossa, as he was again, was briefly imprisoned in Germany before being freed by Martin V in 1418.