For your sermon, it might be a good idea to read 2 Peter 1 in its entirety (rather than just 2 Peter 1:12-15). Also consider reading 2 Peter 3:1–2.
Introduction
Begin your sermon by stating that 2 Peter focuses on God’s promises about Jesus and the future. Talk about what it means for Jesus to be Lord and Saviour, and how Peter wants believers to remember these promises.
Mention that Peter has written this letter as a testament and thus we can be sure that he is focusing on what is of most importance. Perhaps (if it will be helpful) explain how Peter knows that his death is near.
Remembering: nothing new, ongoing process, and active obedience
Explain how remembering focuses on holding fast to the truth that has been revealed.
Apply: give examples of what it will mean in practice to stick with the gospel. Consider some of the temptations to try and look for new mystical experiences or to learn from other books (not the Bible). Explain why Protestant churches have generally had only a pulpit and a table in the church and why.
Explain how remembering is an ongoing process.
Apply: help believers who are familiar with God’s truth to consider how they listen to sermons. Are you still responding to God’s Word by making changes in your life or have you become stagnant because you think you know it all and you are constantly looking for something new?
Explain how remembering involves active obedience.
Apply: give examples of what remembering looks like when it comes to pursuing virtues such as self-control, goodness, perseverance, and love.
Remembering in worship
Explain how gathering for worship and using the sacraments are aids to Christian remembering. When we come to meet with God, we hear the gospel proclaimed once again and we see the promises of the gospel in the sacraments.
Apply: corporate worship on the Lord’s Day is the principal means of grace by which God the Holy Spirit will be at work in our hearts and minds to help us remember. We do not come to worship in order to please the minister, we come to meet with God and have him work in us so that we can remember.
Conclusion
Remembering is vital to the Christian life. We do not begin with the gospel only to move on to better things. Our whole life is one in which we are to be reminded again and again that Jesus Christ is Saviour and Lord and encouraged to live in the light of this truth. Peter has written this letter to highlight the vital importance of this remembering. Let us therefore, by the power of the Spirit, seek to hold fast to what God teaches us in his Word, to reflect with effort on the truth of the gospel—even if we are familiar with it—and to pursue the virtues in which God wants us to walk for his glory.
12 Therefore I intend always to remind you of these qualities, though you know them and are established in the truth that you have.