A collection of papal letters and canons of councils, published in Gaul by an unknown person, Isidore Mercator, or Peccator, about the middle of the 9th century.
The first part contains 60 letters attributed to early popes; 58 of them are forged.
The second part is made up of canons of Councils and the third gives letters of Roman pontiffs, 30 of which are forgeries.
The Decretals were for a long time received as genuine.
In the 17th century they were clearly proved false by Blondel and more completely in the 18th century by the brothers Ballerini.
The object of the writer was not to increase the power of the popes, but rather the authority and independence of bishops against civil rulers.
The authority of the Roman pontiffs was long recognized before the 9th century and is established by facts and arguments independent of this collection of Isidore.
New Catholic Dictionary