A curia is an official body that governs an entity within the Catholic Church.
When the Roman Empire collapsed, many of the administrative functions previously done by the state were subsumed by the only solid institution left, which was the church.
The Bishop and curia took the place of the government officials, often to the point of sitting at the same chair in the same building.
The Curia therefore passed into religious hands, and afterwards changed functions many times but always keeping its traditional name, at least in those Christian denominations that keep a strong continuity with the Apostolic tradition.
[3] The administrative unit of the Holy See is called the Roman Curia, which assists the Pope in governing the Catholic Church.