Kanon, as an ecclesiastical term, has a very interesting history. See Westcott's account of it, On the New Testament Canon, p.498 ff. The original sense, "a straight rod" or "line," determines all its religious applications, which begin with St. Paul's use of it for a prescribed sphere of apostolic work (2 Cor. x.13, 15), or a regulative principle of Christian life (Gal. vi.16). It represents the element of definiteness in Christianity and in the order of the Christian Church. Clement of Rome uses it for the measure of Christian attainment (Ep. Cor.7). Irenæus calls the baptismal creed "the canon of truth" (i.9, 4): Polycrates (Euseb. v.24) and probably Hippolytus (ib. v.28) calls it "the canon of faith;" the Council of Antioch in a.d.269, referring to the same standard of orthodox belief, speaks with significant absoluteness of "the canon" (ib. vii.30). Eusebius himself mentions "the canon of truth" in iv.23, and "the canon of the preaching" in iii.32; and so Basil speaks of "the transmitted canon of true religion" (Epist.204-6). Such language, like Tertullian's "regula fidei," amounted to saying, "We Christians know what we believe: it is not a vague idea' without substance or outline: it can be put into form, and by it we test the spirits whether they be of God.'" Thus it was natural for Socrates to call the Nicene Creed itself a "canon," ii.27. Clement of Alexandria uses the phrase "canon of truth" for a standard of mystic interpretation, but proceeds to call the harmony between the two Testaments "a canon for the Church," Strom. vi.15, 124, 125. Eusebius speaks of "the ecclesiastical canon" which recognized no other Gospels than the four (vi.25). The use of the term and its cognates in reference to the Scriptures is explained by Westcott in a passive sense so that "canonized" books, as Athanasius calls them (Fest. Ep.39), are books expressly recognized by the Church as portions of Holy Scripture. Again, as to matters of observance, Clement of Alexandria wrote a book against Judaizers, called "The Churches Canon" (Euseb. vi.13); and Cornelius of Rome, in his letter to Fabius, speaks of the "canon" as to what we call confirmation (Euseb. vi.43), and Dionysius of the "canon" as to reception of converts from heresy (ib. vii.7). The Nicene Council in this canon refers to a standing "canon" of discipline (comp. Nic.2, 5, 6, 9, 10, 15, 16, 18), but it does not apply the term to its own enactments, which are so described in the second canon of Constantinople (see below), and of which Socrates says "that it passed what are usually called canons'" (i.13); as Julius of Rome calls a decree of this Council a "canon" (Athan. Apol. c. Ari.25); so Athanasius applies the term generally to Church laws (Encycl.2; cp. Apol. c. Ari.69). The use of kanon for the clerical body (Nic.16, 17, 19; Chalc.2) is explained by Westcott with reference to the rule of clerical life, but Bingham traces it to the roll or official list on which the names of clerics were enrolled (i.5, 10); and this appears to be the more natural derivation, see "the holy canon" in the first canon of the Council of Antioch, and compare Socrates (i.17), "the Virgins enumerated en to ton ekklesion kanoni," and (ib. v.19) on the addition of a penitentiary "to the canon of the church;" see also George of Laodicea in Sozomon, iv.13. Hence any cleric might be called kanonikos , see Cyril of Jerusalem, Procatech.4; so we read of "canonical singers." Laodicea, canon xv. The same notion of definiteness appears in the ritual use of the word for a series of nine "odes" in the Eastern Church service (Neale, Introd. East. Ch. ii.832), for the central and unvarying element in the Liturgy, beginning after the Tersanctus (Hammond, Liturgies East and West, p.377); or for any Church office (Ducange in v.); also in its application to a table for the calculation of Easter (Euseb. vi.29; vii.32); to a scheme for exhibiting the common and peculiar parts of the several Gospels (as the "Eusebian canons") and to a prescribed or ordinary payment to a church, a use which grew out of one found in Athanasius' Apol. c. Ari.60.
In more recent times a tendency has appeared to restrict the term Canon to matters of discipline, but the Council of Treat continued the ancient use of the word, calling its doctrinal and disciplinary determinations alike "Canons."