It is often urged that the expression, “it is written,” is enough to prove that the quotation following it is of canonical authority.

“Now with regard to the value of the expression, ‘it is written,’ it may be remarked that in no case could its use, in the Epistle of Barnabas, indicate more than individual opinion, and it could not, for reasons to be presently given, be considered to represent the opinion of the Church.  In the very same chapter in which the formula is used in connection with the passage we are considering, it is also employed to introduce a quotation from the Book of Enoch, [Greek:  peri hou gegraptai hos Henoch legei], and elsewhere (c. xii.) he quotes from another apocryphal book as one of the prophets....  He also quotes (c. vi.) the apocryphal book of Wisdom as Holy Scripture, and in like manner several unknown works.  When it is remembered that the Epistle of Clement to the Corinthians, the Pastor of Hermas, the Epistle of Barnabas itself, and many other apocryphal works have been quoted by the Fathers as Holy Scripture, the distinctive value of such an expression may be understood” (Ibid, pp. 242, 243).  “The first Christian writers ... quote ecclesiastical books from time to time as if they were canonical” (Westcott on “The Canon,” p. 9).  “In regard to the use of the word [Greek:  gegraptai], introducing the quotation, the same writer [Hilgenfeld] urges reasonably enough that it cannot surprise us at a time when we learn from Justin Martyr that the Gospels were read regularly at public worship [or rather, that the memorials of the Apostles were so read]; it ought not, however, to be pressed too far as involving a claim to special divine inspiration, as the same word is used in the epistle in regard to the apocryphal book of Enoch; and it is clear, also, from Justin, that the Canon of the Gospels was not yet formed, but only forming” ("Gospels in the Second Century,” Rev. W. Sanday, p. 73.  Ed. 1876).  Yet, in spite of all this, Paley says, “The phrase, ‘it is written,’ was the very form in which the Jews quoted their Scriptures.  It is not probable, therefore, that he would have used this phrase, and without qualification, of any books but what had acquired a kind of Scriptural authority” ("Evidences,” p. 113).  Tischendorf argues on Paley’s lines and says that “it was natural, therefore, to apply this form of expression to the Apostles’ writings, as soon as they had been placed in the Canon with the books of the Old Testament.  When we find, therefore, in ancient ecclesiastical writings, quotations from the Gospels introduced with this formula, ’it is written,’ we must infer that, at the time when the expression was used, the Gospels were certainly treated as of equal authority with the books of the Old Testament” ("When Were Our Gospels Written?” p. 89.  Eng.  Ed., 1867).  Dr. Tischendorf, if he believe in his own argument, must greatly enlarge his Canon of the New Testament.