教皇不可謬説
In 1869 and 1870 he attended the First Vatican Council in Rome. He made his mark as one of the vocal opponents of the unlimited power of the Pope as well as the doctrine of papal infallibility. He made a speech, completely in Latin, in which he defended protestantism, and which "called forth a storm of indignation from the majority, which finally forced the speaker to leave the tribune." Another speech is a forgery which is falsely attributed to him; in this forged speech it is written against the dogma of Papal infallibility: "Ah ! if He who reigns above wishes to punish us, making His hand fall heavy on us, as He did on Pharaoh, He has no need to permit Garibaldi's soldiers to drive us away from the eternal city. He has only to let them make Pius IX. a god, as we have made a goddess of the blessed Virgin." He later yielded on the issue of infallibility, and he also headed the Slavic deputations to Rome in 1881 and 1888 which succeeded in convincing Pope Leo XIII to allow the south Slavs of Croatia and Dalmatia to retain Slavonic in the Roman Rite liturgy as well as in the Byzantine Rite.