God did not spare the angels who sinned (hamartano), and so He cast them into Tartarus, which was the lowest pit of Hades (2 Pet. 2:4). False teachers are like the false angels, not staying within appropriate boundaries. They have eyes full of adultery, and they cannot quit sinning (hamartia) — they are cursed children (2:14). They are headed for a similar judgment.
The believer who is not growing in grace and virtue is someone who has forgotten that he was previously purged and cleansed from his sins (2 Pet. 1:9). Remembrance of what God did at the beginning is a major help for those who want Him to continue doing the same thing. Justification and sanctification both are received by the instrument of faith.