How do Calvinists understand "anyone" in 2 Peter 3:9? 2 Peter 3:9


  The Lord is not slow in keeping his promise, as some understand
  slowness. Instead he is patient with you, not wanting anyone to
  perish, but everyone to come to repentance


From a similar question in Christianity SE


  It's true that some Calvinists take a different approach and suggest
  that "all" in 2 Peter 3:9 refers only to Christians


So.... God speaks to the people whom He knows that they are Jesus believers :
I am patient with you. 
I don't want anyone from you to perish.
I want everyone from you to come to repentance.

My question is: How come God doesn't want Jesus believers to perish and God wants Jesus believers to come to repentance? Aren't Jesus believers already a repented people (because elected) then impossible to perish?
As the answer you mentioned suggests, the majority of Calvinists and that which is taught by the reputable Wayne Grudem in his Systematic Theology and many others, is that, in 2 Peter 3:9, "all" means "all", and not just "Jesus believers." The verse is definitely referring to salvation, so it is not talking about the heart and mindset of continual repentance that we should all have about all sin. 

It would not make sense for this verse if all was referring to Jesus believers. That seems superfluous, and like you said they are not in danger of perishing, so to speak. 

However, Calvinists could say that verse is referring to the elect, prior to salvation. Because the elect, prior to salvation, would still need to repent and be saved from perishing, but they are the elect from eternity past, so the title of "elect" would be accurate. This is how that interpretation is at least coherent but doesn't adequately explain the use of the term "all" or "everyone" without hermeneutical gymnastics.