The Catholic Church today regards him as an antipope in opposition to Pope Gregory XII, whom it recognizes as the rightful successor of Saint Peter.

Johann Peter Kirsch describes Cossa as "utterly worldly-minded, ambitious, crafty, unscrupulous, and immoral, a good soldier but no churchman".

John XXIII was acknowledged as pope by France, England, Bohemia, Portugal, parts of the Holy Roman Empire, and numerous Northern Italian city states, including Florence and Venice and the Patriarchate of Aquileia; and in the beginning and in 1411–1413 by Hungary and Poland.

Gregory XII was still favored by Ladislaus of Naples, Carlo I Malatesta, the princes of Bavaria, Louis III, Elector Palatine, and parts of Germany and Poland.

Following his election as pope, John spent a year in Bologna and then joined forces with Louis II of Anjou to march against Ladislaus.

There was a huge outcry in Constance when it was discovered that John had fled, and Sigismund was furious about this setback to his plans for ending the Schism.

The King of the Romans issued orders to all the powers on the Upper Rhine and in Swabia stating that he had declared Frederick to be an outlaw and that his lands and possessions were forfeit.

In due course this led to a great deal of political upheaval and many Austrian losses in the region, notably in Aargau to the Swiss Confederation.

In the meantime, Antipope John XXIII and Frederick fled further downriver along the Rhine to the town of Freiburg im Breisgau, which recognised the duke of Austria as its lord.

He convinced Frederick that he stood to lose too much by harbouring the fugitive pope, and the Austrian duke agreed to give himself and John up and return to Constance.

[11] During his absence, John was deposed by the council, and upon his return he was tried for heresy, simony, schism and immorality, and found guilty on all counts.

The 18th century historian Edward Gibbon wrote, "The more scandalous charges were suppressed; the vicar of Christ was accused only of piracy, rape, sodomy, murder and incest.

J.P. Kirsch remarks that "Undeniably secular and ambitious, his moral life was not above reproach, and his unscrupulous methods in no wise accorded with the requirements of his high office ... the heinous crimes of which his opponents in the council accused him were certainly gravely exaggerated.

In 1983 American political satirist and novelist Richard Condon wrote A Trembling Upon Rome, a novel of historical fiction about the life of Baldassare Cossa.