In Canon Revisited: Establishing the Origins and Authority of the New Testament Books, Michael Kruger gives three different threads of evidence for an emerging canon in early Christianity that are found in the New Testament itself.
There are references to canonical books. 2 Peter 3:16, for example, “where Paul’s letters are regarded as on par with … ‘the other scriptures’ of the Old Testament. (Kruger has explained this argument on his blog.)
There are allusions to a bi-covenantal canon. Look at 2 Peter 3:2:
… that you should remember the predictions of the holy prophets and the commandment of the Lord and Savior through your apostles… .
Quoting from Kruger’s remarks on this verse:
Most noteworthy here is the juxtaposition of “prophets’ of the old covenant and the “apostles” of the new covenant as two equal sources of divine authority. This suggests that 2 Peter views divine revelation in two distinct phases or epochs—perhaps an allusion to the beginnings of a bi-covenantal canon. The mention of plural “apostles” would also indicate that any emerging New Testament would be composed of more than just one apostle’s teaching (thus making it clear that Paul is not the only author in view).
Moreover, the fact that “holy prophets” is clearly a reference to written texts suggests that the possibility that the “commandment of the Lord … through your apostles” may also refer, at least in part, to written texts. This possibility finds support in the immediate context, which expressly mentions written apostolic texts (2 Peter 3:1, 16). In addition, the phrase [commandment of the Lord] not only is used regularly to refer to written Old Testament commands, but also finds a notable parallel in Paul: “If anyone thinks that he is a prophet, or spiritual, he should acknowledge that the things I am writing to you are a command of the Lord … ” (1 Cor 14:37). This passage is at least on instance where a “command of the Lord” from an apostle appears in a written text.
There are references to the public reading of canonical books. See Col. 4:16, 1 Thes. 5:27, 2 Cor. 10:9; Revelation 1:3. Why is this important? Portions of the Old Testament were read out loud to the congregation in the Jewish synagogue. The readers of Paul’s letters and Revelation would have known this, and would probably have viewed these letters, then, as “in the same category as other ‘Scripture’ read during times of public worship.”
Other quotations from this book: