Today is the feast of St. Vincent Ferrer, who is primarily known for his missionary work and work in logic. One of the interesting aspects of his missionary work is that a great deal of it was done for the Pope -- but a fact that is often glossed over is that it was the wrong pope. From 1378 to 1418 there was considerable confusion over who was actually Pope. The Popes had been in Avignon for some time, but Gregory XI had moved back to Rome. When he died, the people of Rome, tired of French interference in papal affairs, rioted and demanded a Roman pope; the College of Cardinals didn't really have a Roman who was a serious option, so they elected a Neapolitan instead, Urban VI. Urban was a very strongheaded and stubborn man. He was also a reform-minded man, and this meant that he was a ferocious kind of meddler. Most of the cardinals regretted having made him pope, so they, geniuses that they were, decided to elect another pope, Clement VII, who took residence in Avignon again. This worked out about as well for the Church as you would think. It also created an extraordinary diplomatic crisis across the entire European continent. The map below gives the main lines of division as different parts of Europe recognized either the Avignon pope (red) or the Roman pope (blue) as the true pope:
The map, however, is somewhat simplified, since there were places that switched back and forth as they found it convenient to do so. The diplomatic divisions aggravated the problem, since people now had political interests that gave both sides incentive not to concede anything. When Urban died, the Rome-supporting cardinals elected Boniface IX; when Clement VII died, the Avignon-supporting cardinals elected Benedict XIII. When Boniface died, the Rome-supporting cardinals tried to make peace, on condition that Benedict XIII resign. Benedict refused, so they elected Innocent VII. The structural crisis led to a theological crisis as conciliarism, arguing that a general council would have superiority over any pope, began to look increasingly attractive. Conciliarism would increasingly dominate the scene, even after the overcoming of the Western schism, until it would more or less strangle itself to death through ever more extravagant claims. But while the idea was attractive in the abstract, in practice it just made things worse. Most of the cardinals got together in a council at Pisa to resolve the matter on conciliarist principles; the result was that they declared both candidates no longer pope and elected a new pope. Thus showing, if we needed more proof, that becoming cardinal does not cure stupidity, since all this did was repeat exactly the mistake that had caused the whole problem and turn a dispute between two claimant into a dispute among three claimants. This shifted all the diplomatic alliances again, of course.
One of the Pisan antipopes, John XXIII, convened another council at Constance (Konstanz) in 1414 to discuss how to handle this and other problems. This council ended up being recognized, starting in 1415, by the Roman Pope, Gregory XII. When the council concluded that the only way out was for all claimants to resign, Gregory resigned. And it is worth noting that through the entire process, while the Roman popes never backed down on their claim of being the legitimate pope, they were also the only ones consistently to make concessions for the good of the Church. Martin V was elected pope in 1417, and the Avignon papacy, while still going on, petered out until Clement VIII abdicated in favor of Martin V in 1429.
This is all relevant to Vincent Ferrer, a Spanish saint who lived from 1350 to 1419, and thus saw most of the Western Schism. He was a firm supporter of the antipope Clement VII, and continued to be loyal to the Avignon papacy through the reign of Benedict XIII. He did become disillusioned with Benedict XIII himself, whom he saw as stubbornly refusing to make any effort to repair the damage of the Western Schism, and started preaching against him in 1416. There is a lot of confusion about exactly what went on during this period of his life. I've never seen any actual evidence that Vincent ever regarded the Council of Constance as entirely legitimate; as far as I can tell, he simply seems to have regarded Benedict XIII to be acting in a way that relieved others of obedience to him. A lot of times the story is told in a way that makes it sound as if Vincent accepted Martin V as pope; but this doesn't seem to be the case. He seems to have thought Benedict XIII the legitimate pope to the very end; he just thought that, since Benedict XIII was acting so consistently contrary to the common good of the Church, he had no authority to require obedience.
He was canonized in 1455, after the Western Schism had ended.