In Jude 20-23, the writer addresses three groups of people: doubters, deceived, and departed. The text reads:
But you, dear friends, building yourselves up in your most holy faith and praying in the Holy Spirit, keep yourselves in the love of God, expecting the mercy of our Lord Jesus Christ for eternal life. Have mercy on some who doubt; save others by snatching them from the fire; on others have mercy in fear, hating even the garment defiled by the flesh.
The first group of people Jude addresses consists of those who doubt. That is, they are not able to discern between true doctrine and false doctrine. These may be the same folks Peter describes as “unstable people” that prove to be easy marks for false teachers (2 Peter 2:14). Likely, the doubters are immature believers who are not well-grounded in the faith, although Jude also could be describing unbelievers who are being drawn to Christ, but who must contend with the obstacles of false doctrine. Jude hints that false teachers also prey on disgruntled church members because the false teachers themselves are “discontented grumblers” (v. 16).
False teachers are clever. Often attractive, articulate, and persuasive, they profess to speak for God – even using Scripture and biblical terms – yet they deny the central beliefs of historical Christianity. How can someone seeking the truth, whether an unbeliever or an immature Christian, tell the difference between true doctrine and false doctrine? This is the front line of battle where Jude has challenged us to be, contending for the faith that was delivered to the saints once for all (v. 3).