He was frequently deployed to embassies by Popes Eugene IV and Nicholas V: in 1439, he was sent as legate to Milan and Burgundy, to oppose the claims of the Antipope Felix V; in 1446, he was on a mission to the King of France; in 1451 he went at the request of the emperor as Apostolic Nuncio to Austria. During the period of his nunciature, John visited all parts of the Empire, preaching and combating the heresy of the Hussites; he also visited Poland at the request of Casimir IV Jagiellon. As legate, or inquisitor, he prosecuted the last Fraticelli of Ferrara, the Jesuati of Venice, the Crypto-Jews of Sicily, Moldavia and Poland, and, above all, the Hussites of Germany, Hungary and Bohemia; his aim in the last case was to make talks impossible between the representatives of Rome and the Bohemians, for every attempt at conciliation seemed to him to be conniving at heresy.