Remembering is not something we do once and then it is done; instead remembering is an ongoing process. When Peter writes to these believers, he does not critique them because they do not understand or do not believe properly. He actually tells them that they are firmly established in the truth (2 Peter 1:12). He wants them to continue in this truth and not take it for granted.
Earlier in his letter he told the believers to add knowledge to their faith (2 Peter 1:5)—not knowledge in the sense of knowing some things and now having to learn unrelated things, but knowledge in the sense of having a deeper understanding of what they have already been taught. Growth in knowledge of Jesus Christ who is Lord and Saviour. Growth in the understanding of how this reality connects to all areas of your life. Growth in the ability to see how all of Scripture testifies to Christ (see Luke 24:44).
Peter encourages this remembering because he knows that as human beings we naturally long to have something new. We are excited when we learn new things. For example, consider when someone first becomes a Christian believer—especially those who come to faith as adults. Then there are so many things to learn—lots of new ideas about who God is and what he has done. Indeed every time that person reads a book of the Bible, they will be hearing truth that they have not heard before. It is an exciting time. However, when one has read through the Bible and heard preaching on the same passage for the second or third time, the excitement can wear off. In those circumstances we can be tempted to stop listening because we already know. We reduce Christian growth to learning new things and so when the same truth is repeated we are no longer as interested. Instead of responding in this way, Peter encourages us to keep reflecting on what we hear, even if we might know the truth already, and to keep considering how this truth about God and what he has done ought to change my life.
12 Therefore I intend always to remind you of these qualities, though you know them and are established in the truth that you have.