This is the edition of the correspondence between Mercier and the Holy See between 1887 and 1904, during the crisis where Mercier’s efforts to create an Institute developing a open Thomism were facing internal opposition in the University and was thwarted in Rome by Roman Thomists. The crisis would end with the departure of the Cardinal Mazzela from the Congregation of Studies and his replacement by Cardinal Sattoli. This correspondence is not known: preserved in the archives of the Secretariat of State, Section Belgium, in 1904, it escaped most surveys.