The Decretum Gratiani not only replaced the preceding decrees but also provided a systematic and logical ordering of documents taken from existing collections supplemented by prescriptions of the popes Paschal II (1099–1118) and Innocent II (1130–1143) and of the Second Lateran Council (1139). Until the Code of Canon Law was published in 1917, it remained a standard work for canon law.
Gratian was the first who taught canon law as an autonomous science, although the Byzantine Code of Justinian I had already served as a model in combining civil and religious laws into one code. Canon comes from the Greek word kanon and means a stem or a reed and a long and straight piece of wood, a wooden rule used by masons and carpenters, or a rule with which straight lines are drawn. Figuratively it is the rule of an art or of a trade, a model, a type, or a definitive list or catalog.
With the rise of Christianity, kanon received a new meaning: commandments of God, or in Latin regulae fidei (norms of faith) and regulae morum (behavioral rules). It is in this sense of regulae morum that canon was taken up into law. These behavioral rules began with the Bible and the Didache (Teaching of the Apostles).
As new questions about the faith were posed, heretical and otherwise, church councils and synods were called to answer these questions. This was especially true of the first seven ecumenical councils, which tackled questions on the divinity of Christ, the divinity of the Holy Spirit, the two natures of the one person of Christ, and Mary as the Mother of God, as well as the Council of Trent (1545–63), which answered the many questions of the Reformation.
The answers in the form of decrees would be added on to the list of canons governing behavior of clerics and lay people alike. Over the course of time, as the church grew and branched out, and it became necessary for a rule of conduct to be collected for uniform interpretation and implementation of divine law spelled out in the sources cited. This was the basis of canon law.
Gratian worked with a set method in which three parts may be clearly distinguished. The first part deals with the sources of the law. It also treats subjects concerned primarily with the ecclesiastical hierarchy and the clergy. The second part deals with procedure, secular property, religious orders, marriage, and confession. The last part deals with the rules on the sacraments, except for matrimony, and sacramentals.
Prior to the middle of the 12th century only systematic collections of church prescriptions had existed. With his Decretum, Gratian published the first synthesis of the universally applicable canon law. At the same time he provided the later popes with a foundation upon which their decrees could rest.
In spite of its renown and the great authority of the Decretum Gratiani, it remained a private collection with no universal force of law. The ecclesiastical authorities never officially recognized or approved the collection.