In the first Christian millennium, local and ecumenical church councils issued numerous moral rules, or canons, which were to be obeyed by all Christians in their jurisdiction. Most canons were not considered articles of faith, and could therefore be changed with respect to time and place. Individual bishops and abbots, including many of the Fathers of the Church, also issued rulings on moral and ecclesiastical matters that established a sort of legal precedent by virtue of their saintly authority. Nonetheless, no attempt was made to organize these canons into a body of law, so it was not always clear which of these were binding and which had been superseded by other rulings.
Toward the end of the first millennium, the papacy assumed a more prominent role in matters of ordinary jurisdiction over the entire West, so that papal decretals also came to be understood as legislative acts binding the whole Church. Papal decretals suffered from the same confusion as other sources of law, as it was generally unclear which canons were still in force.
The first systematic attempts to compile all of canon law appeared in the eleventh century. These early works were simply lists of conciliar decrees or declarations of the Fathers of the Church, organized by topic, without attempting to reconcile discrepancies or establish authoritative rulings. The most famous of these compilations is the Panormia
by St. Ivo of Chartres.
In the twelfth century (c.1140-1150), the monk Johannes Gratian composed the first authoritative corpus of canon law, the Decretum Gratiani, 
using the Panormia and various papal decretals as sources. This work was the basis of all later works on canon law, and was the first to receive the title "Corpus Iuris Canonici," though this title was not used until the sixteenth century. Ecclesiastical law was fragmentary in scope, so the Church applied Roman law where no applicable canons existed.
As new papal decretals were enacted, it became necessary to supplement Gratian's work with books of newer decrees. In several cases, these supplements were understood to constitute, with Gratian, a complete body of canon law. The decretals of Pope Gregory IX (1234) was the first book of papal decretals to receive such recognition.
In 1298, Pope Boniface VIII abrogated all canons that had arisen since Pope Gregory's decretals, and replaced them with his own book of decretals, including some from his predecessors. The decretals of Pope Boniface VIII, combined with the Gregorian decretals and those of Gratian, thus became the definitive edition of canon law.
Lastly, in 1317 Pope Clement V added a book of decretals, the Constitutiones Clementinae, most of which are among the canons of the Council of Vienne. The Clementine canons did not exclude other canons that had arisen since Pope Boniface, but the Clementines were the only canons added to the definitive Corpus Iuris Canonici that was first produced with the advent of the printing press in the sixteenth century. The corpus of canon law now consisted of the decretals of Gratian, Gregory, Boniface and Clement.
After the Council of Trent, a new edition of the Corpus Iuris Canonici was completed under Pope Clement VIII, but neither this nor later editions presumed to supplant the source texts. Thus, for most of the modern era, canon law consisted of the canons of the Council of Trent, the body of canons from Gratian to Clement V, plus a confusing array of later papal decretals, resulting in a situation not altogether unlike that of the early medieval period, where it was not always possible to determine which canons were binding. The study of canon law became an arcane field of legal antiquarianism, as experts could not agree on what the law was, much less how to interpret it.
None of the medieval or early modern collections of canons constituted a self-consistent, coherent code of law. Following the custom of Roman law, decrees pertaining to similar topics were listed in the same area, even if they contradicted each other, leaving the jurist to determine which law to follow. The decretals of Gratian mixed legal matters with moral theology without distinction, and simply cited previous rulings and authoritative opinions, without providing guidance on how to reconcile them. For these and other reasons, canonists and clergy implored the Holy See to create a true code of law for the Church.
The first true code of canon law was the Pio-Benedictine Code of 1917, which was a streamlined, organized, self-consistent, and comprehensive body of law. The 1917 code was to be the only universally binding Church law, so ambiguous moral matters found in previous canonical compilations were omitted. Canon law thus became strictly a matter of legality, rather than morality or theology. All previous canons were legally abrogated, but some retain morally binding force if they reflect a teaching of the magisterium on a matter of faith or morals.
The Code of Canon Law underwent one major revision in 1983. Numerous older canons were relaxed or admitted of more cases of dispensation, while the overall structure was altered to present ecclesiology in more egalitarian language. Nonetheless, the monarchical constitution of the Church was preserved and the 1983 Code, when applied with rigor, is hardly more lenient than that of 1917.
Most versions of the Corpus Iuris Canonici may be found online. All texts are in Latin, unless otherwise indicated.
St. Ivo of Chartres (c. 1095) (c. 1140-1150) (1234) (1298) (1917) is now available for purchase. (1983) - English
For more online resources, refer to the list at http://www.ulrichrhode.de/kanon/rq_e.html.
© 2005-2006 Daniel J. Castellano. All rights reserved.