The Postulator is the person who, given the authority of that office, prepared by canonically legitimate formation, and duly approved by competent ecclesiastical authority, represents the Actor / petitioner before the ecclesiastical authority, diocesan as well as Roman (Normae Servandae 1.b: “The petitioner handles the cause through a legitimately appointed postulator.” Normae Servandae 2.a: “The postulator is appointed by the petitioner by means of a mandate written according to the norm of law, with the approval of the Bishop.”). Therefore, he has a double role in the Causes of the Saints: one of “defending” in first person the interests of the Actor / petitioner (who appointed him), and one of collaborating with ecclesiastical authority (who approved him) in the investigation of truth. From this definition of Postulator springs the juridical double role of the postulator, based on the current state of the Cause:

- Postulator of the Cause during the diocesan phase; that is, when he represents the Actor / petitioner before the diocesan ecclesiastical authority;
- Postulator of the Cause during the Roman phase; that is, when he represents the Actor / petitioner before the Congregation for the Causes of the Saints.