The Sacred Congregation de Propaganda Fide, whose official title is "sacra congregatio christiano nomini propagando" is the department of the pontifical administration charged with the spread of Catholicism and with the regulation of ecclesiastical affairs in non-Catholic countries. The intrinsic importance of its duties and the extraordinary extent of its authority and of the territory under its jurisdiction have caused the cardinal prefect of Propaganda to be known as the "red pope".

https://www.newadvent.org/cathen/12456a.htm