Peter seeks to remind them of that which he has just outlined (2 Peter 1:1–11).1 That is the righteousness which is theirs by faith (2 Peter 1:1), the power of God at work in them (2 Peter 1:3–4), the need to pursue virtue (2 Peter 1:5–9), and the promise of a rich welcome into the eternal kingdom of our Lord and Saviour Jesus Christ (2 Peter 1:10–11).
By referring to his letter as a reminder, Peter may also be making use of a classic rhetorical device where it was thought appropriate to assume that the person addressed was already knowledgeable and thus one was simply reminding them. The Christians who receive his letter thus already have the information which Peter is sharing with them (see Romans 15:15; 1 Corinthians 15:1).2
12 Therefore I intend always to remind you of these qualities, though you know them and are established in the truth that you have.