Why did Peter write 2 Peter?

A:

The author of 2 Peter goes out of his way to insist that he is Peter, as he not only names himself Simeon Peter (2 Peter 1:1), but stresses that he was personally present with Jesus on the mount at the transfiguration (2:16-19). The use of the Hebrew 'Simeon' is one of several giveaways that this epistle was not really written by the Apostle Peter, who would always have used the Aramaic form, from his native language - Simon Peter. The Greek-speaking author of 2 Peter was trying too hard to sound like the apostle Peter. (The KJV corrects this error, translating Simeon into English as Simon, but other Bibles keep the original). The author knew about 'all' Paul's epistles (2 Peter 3:15-16) and thought Paul's epistles were already scripture ('other scriptures' - 2 Peter 3:16), neither of which could have been during Paul's lifetime.

The author uses this epistle and Paul's name to attack his own opponents, the 'false prophets', with numerous verbal similarities to what can be found in the New Testament book of Jude. The parallels are so numerous that scholars are virtually unified in thinking that the author has taken Jude's message and simply edited it a bit to incorporate it into his book.

When Peter was actually still alive, there was still eager expectation that Jesus would return soon. With the passing of time that the Christian claim that all would take place "within this generation" (Mark 13:30) and before the disciples had "tasted death" (9:1) started to ring hollow. By the time 2 Peter was written, Christians were having to defend themselves in the face of opponents who mocked their view that the end was supposed to be imminent. So 'Peter' has to explain that even if the end is thousands of years off, it is still right around the corner by God's calendar.
A:

There are two letters attributed to Peter, but New Testament scholars say that neither of them was written by Peter.

First Peter was written from Rome and euphemistically refers to Rome as Babylon (1 Peter 5:13), a clue that allows us to date this epistle to the 80s or 90s of the first century. The Christian and Jewish communities began to refer to Rome as 'Babylon' after Rome destroyed the Jerusalem Temple in 70 CE, just as Babylon had done centuies earlier.

2 Peter was clearly written towards the middle of the second century, although the author goes out of his way to insist that he is Peter, as he not only names himself Simeon Peter (1:1), but stresses that he was personally present with Jesus on the mount at the transfiguration (2:16-19). This epistle was written to oppose Christians of a gnostic persuasion who, from the author's point of view, threatened several ideas the author found central to Christian faith: the prophetic interpretation of the Jewish scriptures, the gospel story of a real Jesus who was also the son of God, the Christian ethic of sexual continence and clean living (holiness), and the apocalyptic view of history.