The apostle always has the possibility of deception in mind and therefore amplifies what he has just said. The deception here is the denial of what follows, which John therefore asserts with great force.
It is a fearful matter when worshippers are made to feel comfortable while still living in the world. We live in an age when false shepherds assure those who have not separated from their past life that God is pleased with them, but the true Christian will be afraid of such dangerous instruction. The failure to give clear warnings against sin is the mark of a false prophet according to John.
Does John deny imputed righteousness? No, he assumes it throughout (1 John 1:7, 9; 2:1-2; 5:4-5), but the error he confronts here is the failure to marry faith and conduct, and therefore he insists that the righteous are only authenticated by their lives.