Peter first tells us to not forget the promises of God, through His prophets, that His Son is coming again (1-2). This is because we Christians are prone to forget this fact. And because in this age we will encounter “scoffers” who would lure us away from faith in those promises (3).
There is a telltale characteristic among these scoffers: they live like they are their own kings, following their own sinful desires (3b). The two always go together. If we knew the horrible destiny to which our lusts lead us - if we could see that and feel it right now - we would be too fearful to sin.
But this never occurs to them - for a double-edged reason. Sometimes our lusts control us to such a degree that we will believe whatever keeps us enjoying those lusts. But for others, that disbelief and scoffing opens the door to the further inflaming of our lusts.
But regardless of which chicken lays which egg, Peter reminds us that the time is short (8-9), and all creation will go through a testing of fire (10). And then we will see what’s what.
Thus God calls us to aggressive, action-filled waiting - a diligent waiting (13-14). This diligence must be toward holiness. And yet it is faith in these promises of a future fire that fires our diligence. Thus the central act of this diligence is faith - faith in the grace of our Lord - the gospel (18). See John 6:29.