I'm presently reading 2 Peter and have come across a couple of points worth noting regarding the nature of the atonement and the possibility of committing apostasy. Arminians affirm the unlimited scope of Christ's atoning work while Calvinists have traditionally affrimed a limited atonement (some are now moving away from this position). Arminians have been mixed over whether or not a true believer can forever fall from grace while Calvinists have always held that those who truly belong to our Lord will not ultimately fall away.
First, in 2:1 Peter launches into a powerful polemic against false teachers. He says of them that they bring in destructive heresy even "denying the master, the one having bought them." This verse could be taken in one of two ways. It might be taken to mean that these persons have never been followers of Christ and continue presently to deny him. If this is the case, then Peter clearly does not hold to a limited atonement. For he says that Jesus has bought these false teachers. On the other hand, it could be describing people who formerly were followers of Christ but have committed apostasy and now deny him. I prefer this reading because it seems that the affirmation of Christ having bought them points to their having experienced a state of grace and right standing. If this is the case, then Peter seems to be affirming the reality that someone might lose their justified status and undermines the idea that all true believers will finally persevere. This reading also carries the implication of an unlimited atonement in that these who have denied Christ and committed apostasy were not beyond the reach of Christ's atoning work having previously been bought by him.
Second, in 2 Peter 3:17 the author exhorts his hearers to be on guard, "in order that you may not, having been led astray by the error of the lawless, fall from your firm position." The NIV renders it, "fall from your secure position." Peter clearly sees the audience as presently secure in their standing with God. They are stable. However, they are not beyond the danger of falling. Whatever sort of security they have, it is not the sort that cannot be lost. For Peter, this has been clearly demonstrated by the false teachers who deny the master who bought them. To say that Peter really means that those who fall away were never truly secure in the first place is to ignore the plain meaning of this text. It is nonsense to speak of those who are not really Christians as secure. Peter believes that those who truly belong to Lord must be on guard and persevere, lest they ultimately fall away.