2 Peter was written in response to some sort of movement from within church which claimed to be Christian, but denied important elements of the faith. They have a overly-realized eschatology and seem to deny the return of Jesus (1:16, 2:1-3, 14, 18). While denial of the return of Christ may seem like a small deviation from the apostolic teaching, it is in fact a denial of the core of Jesus as Messiah, even the Jewish Messiah. This in turn could imply that the opponents reject the standard approach of the apostles to preaching as the Jewish messiah and perhaps a softening on the use of the Hebrew Bible as scripture.
It is possible that 2:5-8 implies sexual immorality, especially since the comparisons to the fallen angels, the time of Noah and Sodom and Gomorrah allude to sexual sins. Even the allusion to Balaam can be seen as a sexual sin since Balaam used prostitution to entice the men of Israel. While someone might suggest that this is just standard heretic bashing, it seems that there is some substance to the charge of immorality since it appears again in Jude and Revelation 2-3.
There are several suggested opponents:
Gnostics. This view is often tied to a later date for 2 Peter primarily because Gnosticism is not a factor until well into the second century. It is difficult to describe a Gnostic theology because it was such a broad movement encompassing many different (and sometimes contradictory) themes. With the exception of a radical realized eschatology and sexual sin, there is little on this list present in 2 Peter. Paul deal with a rejection of the resurrection in 1 Cor 15 and there may be some hints at a realized eschatology in 2 Thess 2. Sexual immorality is too generic to be used to prove 2 Peter is dealing with Gnosticism. At best, Second Peter might be aimed at a sort of proto-gnostic doctrine.
Epicureans. Neyrey suggested that the opponents in 2 Peter are teachers who combine Epicurean philosophy with Christianity. Certainly Epicureanism was popular in the Greco-Roman world, and there are some points of the school of thought that resonate with Christianity But Epicureans were not exactly hedonists, so this may not be a complete answer.
Antinomians. Richard Bauckham suggested that the opponents in the letter represent some form of antinomianism (Jude, 2 Peter, 154-6). “Antinomian” refers to any theology which sees itself as separate from law. For the most part, this takes the shape of permitting (or even encouraging) sinful behavior. These behaviors are not matters of indifference, but rather genuine sin as defined in scripture. Because the believer is free in Christ, they are free to behave however they want, whether that is judged as immoral or not.
The reference in 2 Peter 3:16 to Paul is important – the opponents are “twisting” Paul’s teaching in order to make it say something that was not intended. In my view, this is probably the best way to describe these opponents. They are post-Pauline Christians who have pushed the Pauline doctrine of freedom in Christ well past what Paul did.
The opponents are therefore (in the words of Baukham), “theologically unaware Christians” who compromise with the world on ethical issues (156). This is the point of application to modern deviations from orthodox Christian theology and behavior. How do you deal with the person who claims to be a follower of Jesus yet behaves in a way which is clearly sinful? Do we “shun the unbeliever”? Should we accept them regardless of the sin? How does 2 Peter help with this problem?