For 2 Peter 1:12–15, the focus on the work of Jesus Christ comes from his mention of believers being firmly established in the truth.
The truth which he has in mind is the truth of who God is in Christ, specifically that he is Lord and Saviour. Of importance for your sermon will be to emphasize how this truth is one upon which we are to focus—the truth that we need to remember and allow to change our lives.
12 Therefore I intend always to remind you of these qualities, though you know them and are established in the truth that you have.