Papal infallibility
In 1870, Pope Pius IX's First Vatican Council issued the , which legitimised Papal infallibility and solidified his word as divine. Passy could not accept this assertion of authority, and his family switched to a non-denominational, liberal Protestantism instead. Despite his Catholic background, he was supported by members of different denominations like the Protestant pastor Joseph Martin-Paschoud and Grand Rabbi Lazare Isidor. Passy's son Paul suggested that he may have remained a "liberal Catholic" even after 1870, commenting on his close friendship with the radical Catholic priest Hyacinthe Loyson.