A critical edition of Guglielmo Sirleto's unpublished letters is still missing.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 critical edition of Guglielmo Sirleto's first letters, including other relevant correspondence between Sirleto's arrival in Rome around 1539 and Marcello Cervini's departure for Trent in 1545 by distinguished Roman ecclesiastical officeholders within the papal Curia, aims at demonstrating how the collaboration between Cervini's enterpreneurial endeavors of his printing presses as well as his coordination of important theological brokers during the Dialogues of Religion in Germany, especially at the Diet of Regensburg in 1541, and Sirleto's own ecclesiastical career emerged from a considerable failure at providing an authentic response to Lutheran as well as Protestant theological challenges.

The first volume of the edited correspondence between Sirleto and one of his first most important patron as well as ardent supporters, the Cardinal of Santa Croce Marcello Cervini, offers various hitherto largely disregarded original documents that allow it to perceive Guglielmo Sirleto's first years in Rome as a result from a larger Curial failure in affirming its ecclesiological status and significance towards Lutheran and Protestant theological challenges.