The Council of Ferrara - Florence is convened in Italy and opens on January 8, 1438. It was the continuation of the Council of Basel, which Pope Eugenius IV transferred to Ferrara, then Florence. It was aimed at reunifying the Eastern Orthodox Church and the Roman Catholic Church after the separation which began with the Great Schism of 1054.
A papal bull, Laetentur Caeli, was issued on July 6, 1439 declaring the union of the two churches, however it was short lived and eventually rejected by most of the Orthodox bishops and clergy back in the East, and the reunification ultimately failed due to significant opposition.