"church law," Old English canon, from Old French canon or directly from Late Latin canon "Church law," in classical Latin, "measuring line, rule," from Greek kanon "any straight rod or bar; rule; standard of excellence," perhaps from kanna "reed" (see cane (n.)). Taken in ecclesiastical sense for "decree of the Church." General sense of "standard of judging" is from c.1600. Related: Canonicity.
"clergyman," c.1200, from Anglo-French canun, from Old North French canonie (Modern French chanoine), from Church Latin canonicus "clergyman living under a rule," noun use of Latin adjective canonicus "according to rule" (in ecclesiastical use, "pertaining to the canon"), from Greek kanonikos, from kanon "rule" (see canon (n.1)).
This word is derived from a Hebrew and Greek word denoting a reed or cane. Hence it means something straight, or something to keep straight; and hence also a rule, or something ruled or measured. It came to be applied to the Scriptures, to denote that they contained the authoritative rule of faith and practice, the standard of doctrine and duty. A book is said to be of canonical authority when it has a right to take a place with the other books which contain a revelation of the Divine will. Such a right does not arise from any ecclesiastical authority, but from the evidence of the inspired authorship of the book. The canonical (i.e., the inspired) books of the Old and New Testaments, are a complete rule, and the only rule, of faith and practice. They contain the whole supernatural revelation of God to men. The New Testament Canon was formed gradually under divine guidance. The different books as they were written came into the possession of the Christian associations which began to be formed soon after the day of Pentecost; and thus slowly the canon increased till all the books were gathered together into one collection containing the whole of the twenty-seven New Testament inspired books. Historical evidence shows that from about the middle of the second century this New Testament collection was substantially such as we now possess. Each book contained in it is proved to have, on its own ground, a right to its place; and thus the whole is of divine authority. The Old Testament Canon is witnessed to by the New Testament writers. Their evidence is conclusive. The quotations in the New from the Old are very numerous, and the references are much more numerous. These quotations and references by our Lord and the apostles most clearly imply the existence at that time of a well-known and publicly acknowledged collection of Hebrew writings under the designation of "The Scriptures;" "The Law and the Prophets and the Psalms;" "Moses and the Prophets," etc. The appeals to these books, moreover, show that they were regarded as of divine authority, finally deciding all questions of which they treat; and that the whole collection so recognized consisted only of the thirty-nine books which we now posses. Thus they endorse as genuine and authentic the canon of the Jewish Scriptures. The Septuagint Version (q.v.) also contained every book we now have in the Old Testament Scriptures. As to the time at which the Old Testament canon was closed, there are many considerations which point to that of Ezra and Nehemiah, immediately after the return from Babylonian exile. (See BIBLE ØT0000580, EZRA ØT0001294, QUOTATIONS.)