Peter before his death (which was prophesied almost forty years earlier by Christ; see John 21:18,19 and notes) reminds them they have a permanent reminder of the truth thus he penned this letter.
For we did not follow cunningly devised fables (mythical stories about gods and miracles) when we made known to you the power and coming of our Lord Jesus Christ, but were eyewitness of His majesty. For he received from God the Father honor and glory when such a voice came to Him from the Excellent Glory: “This is My beloved Son in whom I am well pleased.” And we heard this voice which came from heaven when we were with Him on the holy mountain. (Christ’s transfiguration, Matthew 17:2)
And so we have the prophetic word, confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; knowing this first, that no prophecy is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit. (2 Peter 1:16-20)
Peter goes on to warn the people of destructive doctrines, false teachers and their doom. He talks about their depravity and that they will utterly perish in their own corruption. They entice unstable souls and cannot cease from sin. They are accursed children….(2 Peter 15: 1-22)
God’s promise is not slack, be steadfast looking forward to the Lord Jesus Christ’s return. (2 Peter 3:1-17)
Repent, Believe and Trust, the door of your heart by the will of God can be opened by the Lord Jesus Christ. The Holy Spirit will be your guide, your helper, in the triune.