使徒継承
Most of MacColl's writings centre on the question of the "Real Presence" of the Blessed Sacrament and the related issue of Apostolic succession of clergy. Despite limited knowledge of foreign languages, he also maintained contact with continental Roman Catholic dissidents after the First Vatican Council, such as the Croatian, Bishop Josip Juraj Strossmayer of Diakova, and Dr Ignaz von Döllinger in Munich, acting as a discreet intermediary between them and Gladstone.