This passage has quite a few interpretive difficulties in it. I will therefore tread cautiously.
“For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall. For in this way there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ.”
The first difficulty is this: does verse 9 refer to believers or to unbelievers? I argue that he refers to believers here. First of all is the expression “cleansed from his former sins.” Secondly, the terms of the passage seem to indicate that the believer here is living like an unbeliever, rather than that the non-elect covenant member is on a teeter-totter. The following verse seems to me to indicate that the brothers are to receive this warning. Verse 10, by the way, does not indicate that the elect actually could fall from election. Thirdly, the lack of these qualities is not described as unbelief, but as “ineffectiveness, unfruitfulness, nearsighted, blind.” These qualities can more easily describe the believer who is not progressing as he ought, than that “cleansed from his former sins” could describe someone who is not elect.