What does 2 Peter 1:12 mean?

Peter knows two things about his Christians readers. First, they know the truth he has been teaching. In fact, they are rock-solid in those truths. Peter uses the Greek word estērigmenous here, which means "fixed," or "established," or "strengthened." These Christians Peter writes to don't need to know something differently than they already do, in order to live the life Christ.

At the same time, Peter also knows that even Christians who are aware of truth need to be reminded. We need to hear it again and again, to highlight the connection between what we know and what we will do with it. Peter made it part of his life's work to feed the sheep (John 21:15–19) by reminding them of what they know in Christ and urging them to act on it with God's power.

This verse also reflects on Peter's impending death (2 Peter 1:14). Part of his purpose here seems to be reminding the church to hold fast to these teachings, even if Peter is gone. What's been said in prior verses, then, is not meant as a criticism. It's simply Peter giving another loving reminder of what we should be, as Christian believers.