The two main areas of evidence for determining the authorship of the book are internal and external. Internal evidence is based on the contents of the book itself as set forth by the text and includes its self-claims, attributes, and grammatical signature. As we mentioned above, internal evidence is generally less reliable than external evidence. Not because the evidence itself is inherently flawed, but because determining which internal attributes of the book are of significance in relation to authorship and what those attributes imply concerning the author is fraught with subjective assessment.
There is severe danger in relying solely on internal evidence for conclusions about authorship when there is a strong consensus of ancient tradition covering the same. Ones use of internal criteria can and often does become quite subjective, allowing him to prove just about anything he sets out to prove. Sometimes, when there is no such consensus among the ancients, one must rely on internal matters, as is the case with the epistle to the Hebrews. But to use internal evidence to counteract a consistent tradition coming from the earliest period of church history is very ill-advised.1To help the reader more readily appreciate the subjective nature of drawing inferences solely from internal textual evidence, we need only cite the bogus conclusion of Collins in regard to the book of Revelation:
The most significant internal evidence for the date of Revelation is to be found in its references to the destruction of a city called Babylon (14:8; 16:19; 17:5; 18:2, 10, 21). It is highly unlikely that the author of Revelation would have been interested in the conquest of the historical Babylon by the Persians in the 6th century B.C.E. or in the occasions on which it was sacked during the period of the wars among the successors of Alexander. It is even less likely that the author hoped for the destruction of a fortified town called Babylon at the head of the delta of Egypt that was the headquarters of a Roman legion during the early empire. . . . The explanation [by the angel] that follows makes clear that the woman represents the city of Rome. . . . The use of this symbolic name is thus an important indication of the date of Revelation. It implies that the work was written after the destruction of the temple by Titus, that is, after 70 C.E.2To Collins, the mention of Babylon at the time of the author is an obvious indicator that he must be describing Rome, not Babylon. And John must desire the destruction of Rome because the Roman Empire had destroyed Jerusalem by the time of Johns writing. Therefore, the book must have been written after 70 A.D. While this author agrees that the book of Revelation was written later than 70 A.D., this is a precarious position based on a faulty set of assertions! Collins subjectivity in his assessment of the internal evidence is evident. He totally ignores the possibility that this prophetic book (Rev. Rev. 1:3+, Rev. 1:19+; Rev. 10:7+, Rev. 10:11+; Rev. 22:6-7+, Rev. 22:10+, Rev. 22:18-19+) may be describing literal Babylon in the far distant future to Johns time. Thus is illustrated a major weakness in the use of internal evidence: the subjective nature of its application often results in taking a wrong fork in the road of interpretation which renders all subsequent conclusions void. So with internal evidence, we must proceed with caution.