He studied law at the University of Bologna and obtained doctorates in both civil and canon law. He entered the service of Pope Boniface IX, in 1392 and was created cardinal deacon in 1392. In 1403 he was named papal legate in Romagna. He held this post until 1408 when he withdrew his allegiance from Gregory XII. He was a leading figure in calling the Council of Pisa in 1409 which intended to end the Western Schism. Twenty-seven cardinals of both Obediences participated, and hundreds of bishops, generals of religious orders, doctors of theology or canon law as well as ambassadors of every Christian Kingdom. The council ended with the dethronement of the two popes and the election of a new one, Alexander V.. Alexander lived less than a year after his election, and Cossa was elected as his successor and took the name John XXIII. In 1413 he met with Sigismund, King of the Romans who wanted to end the Western Schism. He urged John to call a general council, this time in the free imperial city of Constance. John was the only of the three popes that participated. He promised to resign if the other two would too. He however secretly fled the city on March 20, 1415. Sigismund had him arrested on April 29th. He was brought to Radolfzell where he received news about his deposition by the council. He accepted it and formally renounced his rights to the papacy. Gregory XII resigned and became papal legate for life. Benedict XIII (Pedro de Luna) declined to resign as he considered himself to be the only legitimate pope, because he was the only one who had been named cardinal before the schism. The council deposed him in July 1417, which he ignored until his death in 1423. In November of the same year, Oddo di Colonna was elected as Martin V. Cossa remained imprisoned by Ludwig III of the Palatinate until spring of 1419, when the Medici payed a hefty ransom. After his release, he made his submission to Pope Martin V, who absolved him and named him cardinal bishop of Frascati. He died a few months later and was burid in the Baptistery of the cathedral where the Medici errected a monument for him. Until the mid-20th century, John was sometimes considered a legitimate pope and the last pope of the Great Schism. This however was reinterpreted when Angelo Roncalli chose the name John XXIII.