There have been a number of times when there have been two or more people claiming to be the valid pope. These times were difficult for the Church for obvious reasons, especially when it was not immediately clear who was the validly chosen pope. The most troubling of these periods resulted in what is called the Great Western Schism (1378-1417) and the Council of Constance resolved it.
The crucial background is as follows. For much of the 14th century, the papacy based itself, not in Rome which was somewhat turbulent at the time, but in Avignon in the south of France. This had its advantages but it was deemed important, not least for the stability of the Italian peninsula, for the papacy to return to Rome which happened in 1377 under Pope Gregory XI. The following year he died, and amidst a complicated situation in Rome Urban VI was elected. During the election process there were riots in Rome which may or may not have pressurised the electing cardinals. A few months later, some of the cardinals claimed they had been pressurised and met and elected Clement VII. It is clear that they were also dissatisfied with the policies and style of Urban VI. Lines hardened and created great tensions in the Church, splitting the cardinals and resulting in rival papal courts. The tensions extended to the alignment of the secular kingdoms of the day. Some lined up behind the one claimant and some behind the other. Hostilities happened and the divisions hardened. When each pope died, his supporters among the now divided cardinals elected successors and so on. Urban VI and his successors lived in Rome, while Clement VII and his successors resided in Avignon. That geography was not sufficient to determine the correctness of the situation and the application of canon law was very complex. The situation dragged on, undermining the authority of the Pope, and weakening the Church, and threatening a permanent split in the Western Church. Genuinely holy people, including among them people now canonised, were on both sides.
There was already much debate in this period about the relative importance of church councils and their relationship to the power and authority of the pope. The issues debated by theologians and canonists included to which did most power belong and how often and by whom should general or ecumenical councils be called. This came to be known as conciliarism. These debates acquired a very practical focus as the schism resulting from the double papacy persisted.
It was hoped that a general council might be able to find a solution. In 1409, some cardinals from both sides called a council at Pisa. This resulted in the election of a third pope, Alexander V, but he was not able to persuade the other two popes or their supporters to rally to him. The end result was 3 popes, each with their own group of cardinals and papal court.
It is the great achievement of the Council of Constance that it dealt with this complex situation. It met from November 1414 to April 1418 at the German city on the side of Lake Constance in territory of the Holy Roman Emperor Sigismund, seen as relatively neutral. It was very well attended: overall there were 29 cardinals, 33 archbishops, 150 bishops, 100 abbots, 100 provosts, 300 doctors mainly of theology but also of canon law and the most famous of whom was Jean Gerson, up to 5000 monks and friars, 18,000 ecclesiastics. Around 50,000- 100,000 people visited in all. The Pisan Pope John XXIII was present at first but then fled when he realised he would be deposed. The Roman Pope Gregory XII sent delegates to represent him. Benedict XIII never attended or recognised it, but cardinals from his camp were present. The Council held numerous sessions in that time - 45 in total - to resolve these matters and also address matters of heresy, notably associated with John Wyclif of England and the Czech Jan Hus, and to make some attempts at Church reform. The major actions by which the papacy was united were as follows. In May 1415 John XXIII was deposed as not having been validly elected. Gregory XII agreed to resign for the good of the Church but only after his two delegates formally convoked the council in his name. This gave it real standing and also improved the view of the line of Roman claimants. Attempts were made to persuade the Avignon Pope, Benedict XII, to resign but he refused.
For 2 years the council moved carefully. Benedict XII was eventually deposed by the Council in July, 1417. The Council banned the existing claimants from being re-elected. In the end it decided, in view of the extraordinary situation, to amend the existing electoral rules (determined by the Councils of Lateran III and Lyon II). It created a broader conclave to try and ensure consensus and also support afterwards for whoever was elected. As well as the cardinals, figures would represent the various powerful nations. The winning candidate would need a two-thirds majority of each group. With this agreed, within 3 days, the expanded conclave chose Martin V in November 1417. He was accepted, ruled until 1431 and reunited the Church, ending the schism. Once elected, he strongly asserted Papal authority in a private consistory in March 1418, shortly before the formal closing of the Council. Although other residences were offered, he finally chose to go back and reside in Rome.
This resolved the schism but where did it leave the issue of conciliarism? An early decree Haec Sancta Synodos claimed authority for a general council as coming directly from Christ and so it required the obedience of all including the popes. However, these early sessions, before Gregory XII formally called it, were not considered valid by the popes who emerged as validly accepted, and its decrees were not approved. In 1417, a month before the election of Martin V, Constance passed the decree Frequens. It stated that a general council should either always be in session or expected soon. It went on to mandate that a General Council meet 5 years after Constance, and one 7 years after that, and then every 10 years in perpetuity. Popes could shorten the gap but not lengthen it. Although Martin V strongly asserted his own authority upon his election and the Papal practice was only to accept conciliar decrees and decisions that it formally approved, nonetheless the Church and the papacy had to live with the ongoing concrete reality of what was variously seen as the right or demand to have regular General or Ecumenical Councils. This shaped the ongoing history of the Church in the 15th century as we shall see.