This section of 2 Peter includes the final paragraph of the letter opening (1;12-15) and the first section of the main letter body (1:16-21).
In 1:12-15 states his reason for writing: to leave behind a permanent reminder of the true apostolic gospel. His impending death makes it crucial that this letter serves to stir up his readers to remain faithful to the truth long after he is gone.
The opening paragraph of the letter body (1:16-21) asserts the validity of the apostolic gospel. Peter uses two witnesses (cp. Deut ): the eyewitness testimony of the apostles (1:16-18) and the OT (1:19-21). Together these two witnesses confirm the truthfulness of the gospel.
Want to learn more? You can listen to the audio and follow along with the handout below: