The letter begins with a description of our call to follow Jesus (3-15). Notice we are given "great promises" that allow us to "escape from the corruption of the world (4)". What intrigues me is that what follows requires choice and effort on my part. He even uses the phrase "For this very reason, yo must make every effort . . . (5)". This is the great paradox of the Christian journey. God has given us the gift of salvation, of new life, of eternal life, and of abundant life . . . it is a gift, it is not earned or deserved. But this gift is just the beginning of the journey. The goal of the journey is to become like Jesus and that requires constant choice and effort on our parts. The point of the journey is to be transformed -- remade in the image and likeness of Christ. It requires constant reminders (12). This is why reciting prayers like the Lord's Prayer or the creeds is an important part of our life in faith. Reciting the creed reminds us of the things in which we have come to trust and believe -- and, frankly, we need to be reminded.
The second half of chapter 1 (16ff) is a report of the experience Peter had on the mount of transfiguration (see Mathew 17). The author is reminding his readers that he did not just make this stuff up, but, instead, was there, saw the glory, heard the voice of God -- he actually knew Jesus. Somehow that changes things.