10 But the day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything done in it will be laid bare. 11 Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives 12 as you look forward to the day of God and speed its coming. That day will bring about the destruction of the heavens by fire, and the elements will melt in the heat. 13 But in keeping with his promise we are looking forward to a new heaven and a new earth, where righteousness dwells.
As Peter finishes his 2nd letter to the church, he discusses the day of The Lord. This is frequently referred to as a day of judgment, and with Jesus' teachings his own return to judge the living and the dead. Of course Peter doesn't know more any more than we do as to when that day is going to come. But what he does know is that it is going to come in a way which he describes as a thief in the night. What is the one thing about a thief coming at night? It is when people are most likely to be asleep.
Then Peter describes the totality of what the end will bring. The heavens will disappear with a roar. Notice "heavens" is plural. Also that the heavens are part of the created order. Not only will heaven disappear but the earth will be laid bare. Later it says both heaven and earth will be destroyed by great fire. Fire has always been a sign of judgment in the bible.
Next he urges them to lead holy and godly lives. The best way to prepare for Jesus' return is to be living the type of life he commanded while he was here. Finally he tells them not to be afraid but to be confident that God keeps His promises. This is the same promise Peter is looking forward to, which is why he says "we". Notice he says, "looking forward to a new heaven and new earth". While the day of The Lord will come like a thief, we don't have to be afraid. We can look forward to it because of what Jesus has done for us. We have complete theft protection because of Jesus' blood and righteousness. Though this heaven and earth will pass away, we have an ever better and new heaven and earth on the way. "Our citizenship" as Paul says, "is in heaven". But while we wait for his sure and certain return, we rely on His promises and live the life He called us to live.