Ever since God has been expressing His truth to human beings, Satan has tried to undermine Divine authority by twisting truth into lies (Genesis 3:1-5).  The Old Testament prophets were appointed because God’s appointed truth-tellers (the kings) had been caught up in the lies, believed them and allowed them space to infiltrate what Israel believed.  Additionally, there were other false prophets who seduced the hearts of God's people, like Balaam (Revelation 2:14).  In the same way, Peter says, false prophets were lurking in the churches.  Appearing to be followers of Jesus, they were enemy agents.  They may not have thought of themselves like that, and they would probably have come across as sincere teachers.

Peter contrasts them with the true prophets whose words have been included in Scripture (1 Peter 1:19-21).  The motives and words of the false teachers were not from God (Jude 1:12-13).  Unlike the true prophets whose message was open to all who would listen, the counterfeit preachers behaved covertly.  They packaged their lies in sufficient truth to deceive the believers who were unaware that they were being seduced away from Christ.

Their false teaching was focussed on the person and work of Christ.  They ignored His deity and the all-sufficiency of His sacrifice.  The danger to the church was, and still is, that whenever error is given house-room and not immediately rejected (Ephesians 4:25-27), it ruins believers' discernment and spoils their consciences (1 Timothy 4:2) - so that people start to believe the lies.  Peter accurately predicted that such teaching would eventually destroy churches and bring the gospel into disrepute.  Peter himself had been a victim of such deception and Jesus rebuked him (Matthew 16:21-23), and later Paul needed to challenge Peter to come back to the true gospel (Galatians 2:11-21) – these were important lessons which Peter learned well.

Unfortunately, false teachers are still around.  They are often popular, gathering admiring crowds who congratulate 'wise insights' (1 Timothy 4:1-4).  Even firm believers may be fooled.  So how can the church be defended against such doctrinal perversion?  Firstly, we must not be gullible: be on guard, aware that wherever the Lord is working in truth, error will lurk in somebody who speaks persuasively awaiting the chance of a hearing.  Secondly, we need to guard the apostolic teaching about the truth of Jesus Christ, the Son of God, the Saviour of the world (Acts 20:28-31).  And the best way to protect true doctrine, is to preach it!  Thirdly, do not be fooled into wrong belief just because thousands of other people think it is right (Luke 6:26): check it carefully against the teaching of the apostles who were appointed by Jesus to define and proclaim the truth of the gospel.  Then, whenever you find anything else taught, be bold and brave in identifying the error and warning others against accepting and believing it.