During the First Mongol invasion of Hungary in 1241–42, the Diocese of Eger, along with the town and its cathedral suffered severe damage, while the episcopal treasuries were looted. Cletus Bél managed to survive the invasion, but his whereabouts during the events are unknown. It is plausible that he did not belong to the accompaniment of Béla IV of Hungary, who took refugee in Dalmatia. Cletus had to reorganize the church institution in his diocese; he requested Béla IV to transcribe and confirm the privileges of the bishopric. In 1245, the vicars of the diocese complained to Pope Innocent IV that Cletus, the chapter, as well as the archdeacons, had confiscated all the church income for themselves, which made their livelihood impossible. The pope appointed subdeacon Martin to investigate the case and called the bishop to treat the vicars fairly. Cletus was last mentioned as a living person on 12 December 1245. He died still in that month, as his successor Lampert Hont-Pázmány was already styled as bishop-elect in 1245. According to a later document, Cletus granted nobility of church ("nobilis ecclesiæ") to a certain Samuel, son of Sibin. However, after Cletus' death, this Samuel gathered and set fire to the formerly reissued privilege letters of the diocese for unknown reasons. Avoid retaliation, Samuel fled to Ruthenia. Greek Catholic priest and historian János Szarka considered the Diocese of Eger initially followed Byzantine (or Greek) Rite which, however, gradually switched to the Latin rite during the episcopal activities of Thomas and Cletus. Accordingly, the latter had already rewritten the privileges of the diocese according to the new rite after the Mongol invasion. Szarka argued Samuel was a cleric who practiced Greek rite, and this was the reason for the destruction of the diplomas.