When Peter refers to Scripture, it is natural to assume that he has in view what we know today as the Old Testament. But there are indications within 2 Peter that he understood God’s word as having a twofold structure that anticipates what today we refer to as the Old and New Testaments.
What 2 Peter 3:2 Has to Say
The first passage to note is 2 Peter 3:2, where the apostle exhorts believers to “remember the predictions of the holy prophets and the commandment of the Lord and Savior through your apostles.” The “predictions of the holy prophets” refers to the Old Testament in its totality, evidenced by Peter’s use of specific texts from all three major divisions of the Hebrew canon (Torah, Prophets, and Writings). As he had written previously, Peter was convinced that the Old Testament authors were serving present-day believers as they proclaimed the good news that God had revealed to them (1 Peter 1:10–12, 22–25). Since the expression “predictions of the holy prophets” refers to written texts, there is good reason to conclude that the parallel expression “the commandment of the Lord and Savior through your apostles” also refers (at least in part) to written texts (cf. Jude 17). The plural “apostles” further indicates an awareness of multiple divinely inspired and authoritative writings, including Peter’s own letters.
What 2 Peter 3:16 Has to Say
The second text is found in the same chapter. Peter explains that “ignorant and unstable” people twist Paul’s letters “to their own destruction, as they do the other Scriptures” (2 Peter 3:16). Peter is not only aware of multiple Pauline letters but assumes that his readers are likewise aware of them and likely have access to them in some form. Even more significantly, Peter acknowledges that Paul’s letters are regarded as inspired Scripture, of equal authority with the Old Testament writings.
Peter Assumes We Share His View
Perhaps most noteworthy is that Peter can make such statements almost in passing. He makes no attempt to explain or defend the twofold nature of Scripture consisting of the Old Testament writings alongside the writings of the apostles. At the least this demonstrates a realization very early within the history of the church that God was speaking authoritatively through his apostolic witnesses as they wrote. The close association between God making a covenant with his people and then inspiring authoritative written documents in connection with that covenant is well-established in the Old Testament. Recognizing that Jesus had inaugurated the new covenant with them, it makes sense that he would inspire authoritative written documents that explain that covenant and lay out the stipulations for how his covenant people should live. Peter even frames this current letter as a “reminder” (2 Peter 1:13; 3:1) to enable the readers “at any time to recall these things” (2 Peter 1:15).
Thus there is every reason to see in what Peter says an early recognition of the twofold nature of Scripture, consisting of the Old Testament and the apostolic writings.
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 In 2 Peter 1:20 the apostle states that “no prophecy of Scripture comes from someone’s own interpretation.” That claim comes on the heels of describing Jesus’s transfiguration, in which he alludes to or echoes texts from the Torah (Genesis 22), the Prophets (Isaiah 42:1), and the Writings (Psalms 2, 8).
 Michael J. Kruger, The Question of Canon: Challenging the Status Quo in the New Testament Debate (Downers Grove, IL: InterVarsity Press, 2013), 150.
 On this point see especially Meredith G. Kline, The Structure of Biblical Authority, 2nd ed. (Eugene, OR: Wipf & Stock, 1989), 43–75.
 For elaboration on this point, see Kruger, Question of Canon, 43–78.