From April 19th to 22nd 1927, the Congrès d'histoire du christianisme was held in honor of Alfred Loisy, who was celebrating his 70th birthday at the time. At fi rst Franz Cumont, Loisyes longtime friend, belonged to the congresses Honorary Committee, but ultimately he withdrew his support. In search of the reasons for this withdrawal, I study the correspondence between Loisy, Cumont and Paul-Louis Couchoud, organizer of the event. I thus discover a discussion about the competence and ideological stance of the researcher active within the fi eld of the history of Christianity in France in the fi rst half of the 20th C, a debate closely related to the question of the position of the study of Christianity within the discipline of the history of religion.