One question that very easily arises from 2 Peter 2:20–21 concerns whether or not Christians can lose their salvation. Is it possible for someone who belongs to Jesus Christ to fall from grace?
A detailed answer to this question can be found in chapter 5 of the Canons of Dort. It is worth reading that chapter in its entirety.
In brief: Peter has already mentioned that God protects and guards those who belong to him (2 Peter 2:5, 2 Peter 2:7), having given us everything we need for life and godliness (2 Peter 1:3–4) so that we can persevere in faith to the very end, thereby making our calling and election sure (2 Peter 1:5–11). God will keep his people. The fact that there are some who do not persevere does not cause us to doubt God’s power, but is a confirmation that those individuals were never truly part of God’s people (see 1 John 2:19). True believers who have been united to Christ through true faith will not commit apostasy.
20 En effet, si après s'être retirés des souillures du monde, par la connaissance du Seigneur et Sauveur Jésus-Christ, ils s'y engagent de nouveau et sont vaincus, leur dernière condition est pire que la première.