What does Jude 1:19 mean?

Earlier in this letter, Jude warned that false teachers were a deceptively-hidden danger among true Christians (Jude 1:12). Along those same lines, according to this verse, false teachers cause trouble in the churches, leading to church splits. Since they appear to be legitimate believers but are not (Jude 1:13), their influence can lead other believers astray.

The apostle Paul warned a young pastor, Titus, about disruptive apostates. He writes: "For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. They must be silenced, since they are upsetting whole families by teaching for shameful gain what they ought not to teach" (Titus 1:10–11).

According to Jude, these unbelieving deceivers think and act like "unreasoning animals" (Jude 1:10). They follow their sinful natures, proving even more clearly that they lack the Holy Spirit (Ephesians 2:1–3; Romans 8:9). This is important, since Jude is not merely referring to confused or mistaken believers. He is pointing out that false teachers, like those so far described, are not actually believers at all.