During the high Middle Ages, in the context of discussions concerning the union between the Western and Eastern Churches, at the Second Council of Lyon and at the Council of Florence, the Catholic doctrine of Purgatory was defined. At the beginning of the modern ages, faith in postmortem purification was first contested by Protestant theologians and it was also condemned because it was impossible to consolidate it with Luther’s christological thesis of justification. In the sessions of the Council of Trent dealing with the demands for reform of the Church and with many doctrinal questions, the Council’s father had to face questions respecting Purgatory in connection with the doctrine of justification and with the sacrificial character of the mass. In the context of the Doctrine of Justification, a dogmatical sentence was approved. The Decree on Purgatory was revealed from the discussions during the last session. The Doctrine of postmortem purification is summarized and the disciplinary directions for bishops concerning propagation of this faith in between believers are stated.