1 John 3:8
 Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil. (ESV)
I am convinced that one of the biggest lies of the enemy is that mankind is basically good. In fact, many churches are filled with people who believe such drivel. As a result, they never surrender their lives to Jesus Christ, repenting of their sins, and receive salvation. How tragic to think that people could be in the very place that should tell them of their sin nature and their great Savior, yet that truth is withheld from them.
The Apostle John was dealing with similar deception in the church. The false teachers taught that their habitual sinning was no big deal. They claimed spiritual purity while practicing the very opposite. John makes a strong rebuke by reminding them that if their continual practice is sinning, they are proving that they are not only unholy, they are in fact showing the nature they inherited from their true master, the devil.
I dare say, many people in our society would take great offense at being told that they were of the devil. Try telling your neighbor or co-worker that they are of the devil. John is talking about more than just false teachers. He indicts all of mankind that does not know Christ. All of those who do not know the Savior practice sin. And that practice of sinning is of the devil. Here we are reminded of the Apostle Paul's words to the Ephesians:
 in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— (ESV)
Do you find your identity in a lifestyle of rebellion and sin? Surrender to Jesus. Do you find your identity in submitting your life to Jesus? Continue to walk with him. Allow him to use you to bring good news to those who are "of the Devil," as you once were.