Decretals, letters sent by the early popes to different churches, containing decrees deemed necessary for the maintenance of discipline or the good of religion. The term relates to the Decretum of Gratian, consisting of decrees, and forming the chief body of church laws. The collection or code of Dionysius Exiguus, a learned Scythian monk (died about 540), was published in Rome. Its second part is made up of the decretals of several popes, the earliest of whom is Siricius (384). This code, with the additions made by successive pontiffs, was presented to Charlemagne by Adrian L, and this copy is known as Codex Canonum. About 845 appeared the false Isidorian decretals, a collection made as some think in Spain, according to others in the west of Gaul. It was an attempt to codify scientifically the existing church laws, and contains three classes of documents: some perfectly genuine, and attributed to their real authors; others substantially so, but published under the names of councils and popes to whom they did not belong; and others altogether spurious.

Beveridge has conclusively shown that all the decrees or letters thus falsely attributed to certain authors were mere tissues of texts selected from the canons of councils, epistles of popes, and works of ecclesiastical writers, especially of the 5th and 6th centuries. This pseudonymous author, Isidore Mercator or Peccator, is not to be confounded with Isidore of Seville (died in 636), whose code continued up to the 12th century to be the basis of ecclesiastical law in Spain, and was held in great reverence throughout the western churches. In 1151 appeared the compilation of the Benedictine monk Gratian, entitled at first Concordantia Discordantium Canonum, and afterward called Decretum Gratiani. In 1234, under Gregory IX., was published the collection of Raymond de Pennafort, comprising five books of decretals. This is the first complete body of ecclesiastical laws given to the world with the approbation of the Roman see. To these five books Boniface VIII. added the Sextus Deere-talium, or the "Sext," in 1298; in 1308 were added the "Clementines," or constitutions of Clement V.; and in 1317 the Extramg antes of John XXII., embodying decrees and laws which had no fixed place in the code. The Extravagantes communes were added by later pontiffs.

These decretals, including the Decretum of Gratian, form the Corpus Juris Canonici, and comprise all subjects which in that age were within the cognizance of the ecclesiastical courts, as the conduct of the clergy, matrimony and divorce, inquisition of criminal matters, purgation, penance, excommunication, etc. (See Canon Law).