The Codex epistolaris Carolinus is a collection of 99 letters from reigning popes to Carolingian rulers written between 739 and 791.
[3] The preface, or "Praefatio", of the collection states that the letters were compiled as the original parchment detailing the correspondence had been "partly destroyed and erased" by "great age and carelessness".
Historian Dorine van Espelo believes that the compiling of letters to form the Codex Carolinus is representative of this religious reform and Charlemagne's commitment to orthodoxy and the Catholic Church,[5] as letters from the papacy would contain religious instruction directly from the most authoritative and orthodox Catholic source.
[6] The Codex being formed in a time when the Frankish Court was concerned with Christian orthodoxy explains why the collection includes three letters sent from pope Hadrian to Spanish bishops in response to the growing Adoptionist heresy.
[10] Achim Thomas Hack's work, titled Codex Carolinus, has done much to reignite scholarly interest in the collection through his analyses of individual letters and contextualising them within diplomatics and epistolography.