What does 1 John 3:4 mean?

In contrast with the purity described in verse 3, John labels the "practice of sinning" as "lawlessness." This is strong, spiritually-loaded terminology. In the Bible, those who do not know Christ are called "workers of lawlessness" (Matthew 7:23). Unbelievers are marked by lawlessness (Matthew 23:28), and will increase in the end times (Matthew 24:12). Lawlessness was associated with those who broke the Law of Moses (the Torah) and were therefore sinners.

Romans 6:19 and 2 Corinthians 6:14 also describe unbelievers as those involved in lawlessness. In the last days, the Antichrist, a man of lawlessness, will come (2 Thessalonians 2:1–12). Christ, however, gave Himself "to redeem us from all lawlessness and to purify for himself a people for his own possession who are zealous for good works" (Titus 2:14).

At the very least, John is dispelling the claim that the gospel is a license to sin. Much as Paul does in the books of Romans and Galatians, John indicates that a true walk with Christ leads a person to sin less. Sin is sin, and even though believers are capable of sinning, the presence of Christ means they cannot do so lightly.

The false teachers John spoke against likely had a weak view of sin, considering holiness unimportant. John clearly notes that those who know God will seek to live for Him, including a life that increasingly flees from sin and follows the teachings of Christ.