A major divide exists among scholars on the precise understanding of idias epiluseos ["from an individual's own interpretation," or "by the will of man"]. Some, such as Kelly, assert that this verse forbids the private interpretation of Scripture by the reader (or hearer) outside of an authority such as the church. Thus, idias, "from an individual's own," would refer to any reader of Scripture, rather than to the prophet who authored Scripture. Along with epiluseos, "interpretation," these two words would pertain to any person's unauthorized, illegitimate interpretation of written Scripture.
However, that understanding of idias epiluseos does not make sense in the present context. In 2 Pet 1:16-18, Peter addressed the divine origin of the apostolic message. 2 Pet 2:21 addresses the same issue of origin regarding Scripture in general. Moreover, 2 Pet 1:21 includes the explanatory gar, "for," which draws close connection between 2 Pet 1:21 and 2 Pet 1:20, implying that Peter's further declarations about the inspiration of Scripture in 2 Pet 1:21 are intended to elaborate upon his statements in 2 Pet 1:20. Thus, 2 Pet 1:20 too must be about the origin and inspiration of Scripture, not about its later interpretation by readers. Since the context of 2 Pet 1:20 addresses Scripture's divine origin, and since idias epilueos ["by the will of man"] in 2 Pet 1:20 supports this topic if taken to refer to a prophetic author (rather than a later reader), the best conclusion is that 2 Pet 1:20 speaks of the divine origin of Scripture as well. C. Giese, 2 Peter and Jude (Concordia 2012), 93-94.