The word "canon" comes from the Greek kanon, which in its original usage denoted a straight rod that was later the instrument used by architects and artificers as a measuring stick for making straight lines. Kanon eventually came to mean a rule or norm, so that when the first ecumenical council--Nicaea I--was held in 325, kanon started to obtain the restricted juridical denotation of a law promulgated by a synod or ecumenical council, as well as that of an individual bishop.
In the Eastern Orthodox Church, canons "are ecclesiastical norms issued by the Church throught the collective voice of the bishops gathered in ecumenical or local synods, speaking throught the inspiration of the Holy Spirit and in agreement with Christ's teaching and the dogmas of the Church. In addition, the Fathers of the Church issued canons or wrote letters that eventually came to be considered entirely or partially canonical. The autocephalous churches issued decrees concerning the life of the Church and particular aspects of their order and discipline. The Tradition of the Church also contributed to the corpus of ecclesiastical norms with truth of faith that eventually entered into universal acceptance. A special place in Canon Law is given to the [eighty-five] Canons of the Holy Apostles, attributed to the Apostles and collected in different works."
Greek kanon / Ancient Greek: ,Arabic Qanun / , Hebrew kaneh / , "straight"; a rule, code, standard, or measure; the root meaning in all these languages is "reed" (cf. the Romance-language ancestors of the English word "cane"). A kanon was the instrument used by architects and artificers for making straight lines.
Some writers think that the Church preferred the word canon to law, as the latter had a harsh meaning for the faithful in the times of persecution.
The early Fathers use canon as equivalent to the rule of faith, or for some formula expressing a binding obligation on Christians.[note 1]
Bickell declares that for the first three hundred years, canon is scarcely ever found for a separate and special decree of the Church; rather does it designate the rule of faith in general. He appeals to the fact that the plural form of the word is seldom used in the earliest Christian writers.[note 2]
With the fourth century began the use of canon for a disciplinary decree, owing to its employment in this sense by the First Council of Nicea (325 A.D.).
The Cassinese editors of Ferraris (s. v. Canones) say that in the first ages of the Church many disciplinary regulations were not required, and hence it was scarcely necessary to discriminate decrees into dogmatic and disciplinary, as the faithful classed both under the obligation to observe the general rule of faith.
The word decretum during the same period, though signifying in general an authoritative statute or decision, began to be limited more and more to dogmatic matters, while canon when used in opposition to it was restricted to laws of discipline. That this usage, however, was not invariable is evident from Gratian's use of "Decretum" to signify his collection of canons and decrees.[note 3]
As ecclesiastical regulations began to multiply, it became necessary to gather them into codices, which generally received the title of "Collection of Canons". In these, civil laws are often added to the Church regulations. For such collections the Greeks used the term Nomocanones. The Latins have no special name for them, though Capitularies (q. v.), e. g. of Charlemagne, is sometimes referred to as a somewhat parallel usage in the West.
The Eastern Orthodox Church, principally through the work of 18th-century Athonite monastic scholar Nicodemus the Hagiorite, has compiled canons and commentaries upon them in a work known as the P?dálion (Greek: , 'Rudder'), so named because it is meant to "steer" the Church in her discipline. The dogmatic determinations of the Councils are to be applied rigorously since they are considered to be essential for the Church's unity and the faithful preservation of the Gospel.
With the Council of Trent in the sixteenth century began the departure from this ancient usage. This council used the word canon for short, dogmatic definitions with an anathema attached to them. On the other hand, it gave the name of decrees to its disciplinary regulations. The example set by Trent was followed by the First Council of the Vatican.
The usage of Trent seems to bring canon nearer to the signification it bore before the First Council of Nicea, when it referred rather to faith than to discipline. The general idea of a decision by Church authority seems to be also the root-meaning of the expressions "Canon of Scripture", "Canon of the Mass", "Canon of Saints", although for the last term Ducange (s. v. Canonizare) suggests a somewhat different origin.
As to the authority of ecclesiastical canons in the Catholic Church, a distinction is made when speaking of canons of faith and canons of discipline, for the former are irreversible, the latter are not. Similarly, canons containing a precept already binding by reason of Divine or natural law, cannot be on the same footing as those that are of mere ecclesiastical origin.
In general, the Corpus Juris Canonici declares[note 4] that canonical statutes are binding on all; likewise[note 5] that bishops are the guardians of the canons and must see to their observance. When there is question of canons in the ordinary ecclesiastical sense (namely, that which obtained before the Council of Trent), as they refer principally to matters of discipline, it must be borne in mind that they are neither immutable nor irreformable.
The subject-matter of such canons depends not only on circumstances of persons, places, and times, but also on considerations of expediency or temporary necessity. A change in any of the causes which brought about the framing of the canons, will make a change in their binding force, for disciplinary regulations are almost necessarily mutable.
In like manner when there is question of the binding force of a canon, it is important to determine whether it was issued by a general council or by the decree of a pope, as imposing an obligation on all the faithful, or whether it was framed solely for restricted regions or persons. In the latter case its binding-force is as restricted as its scope.
The Catholic Encyclopedia argues that the object which the Church has always had in view in promulgating her canons has been the guidance and preservation of the clergy and laity in the duties of a Christian life and in the best methods of ecclesiastical administration.
Although, therefore, such canons contain elements of positive human law, yet ultimately they are founded on the Divine or natural law. As such, they cannot be entirely abrogated by contrary custom (Ferraris, loc. cit.), though their rigour may be mitigated by certain circumstances, on the ceasing of which, the pristine rigour of the canon would be again binding. When they are entirely of human law, they may, of course, be completely abrogated, not only by legislation on the part of the proper authorities, but also by legitimate custom.
William H. W. Fanning, in his 1913 Catholic Encyclopedia article on Ecclesiastical canons, suggests that
The neglect of the prescriptions of the sacred canons has always been the source of corruption in morals, and perhaps the chief reason for the loss of faith by nations as well as by individuals.
According to Fanning, the study of the sacred canons is especially enjoined on the clergy. Fanning speculates that perhaps most of the regulations refer directly to ecclesiastics, and suggests that clergy will find in them the surest guidance for their own conduct and for the fruitful exercise of their ministry in directing the faithful.