A critical edition of Guglielmo Sirleto’s unpublished letters is still missing. The partial transcriptions of this large collection of epistles in the volumes of the Concilium Tridentinum by Gottfried Buschbell only provided partial transcriptions relating to Cardinal Marcello Cervini’s legation during the Council of Trent betwen 1545 and 1547.While scholars have recognized Sirleto’s assistance to Cardinal Cervini’s Tridentine exploitations and compositions of various critical decrees, the larger context of his entry into the household and Roman enterprises oft he powerful cardinal editore Cervini remain largely obscure. The following volume of registered letters corresponding to the first volume of the edited correspondence aims at complementing the edited volume of epistles by including annotated editions of largely unknown manuscripts by distinguished Roman ecclesiastical officeholders, such as the papal librarian Agostino Steuco, addressing the theological issues discussed at the Dialogues of Religin in Germany and which were later reprossed in the published first editions printed by Cardinal Cervini’s Latin and Greek presses in Rome.The volume of registered letters provides a useful instrument in not only offering concisive descriptions of the edited collection of epistles, but offers also valuable materials for future scholarship in reconstructing the intellectual atmosphere at the Roman Curia and Cardinal Marcello Cervini’s agency in strategically manipulating this new group of theological as well as ecclesiastical scholars.