Trianon meant the destruction not only of the Hungarian state, but also of the Hungarian ecclesiastical organisation (dioceses and the various monastic provinces). The issue affects relations between Hungary and the Holy See in many ways, but also goes beyond them in several dimensions.
On 23 June 2021, the Vilmos Fraknói Vatican Historical Research Group held a conference on the Church Historical Consequences of the Peace Treaty that ended World War I. The written version of the lectures presented at the conference, edited by György Sági and Péter Tusor, was published as volume 5 of the Collectanea Studiorum et Textuum series under the title "Hungarian Catholicism and Trianon".
The full volume can be accessed and downloaded here.