What does 1 John 2:6 mean?

This verse completes John's thought which was begun in verse 5. Anyone claiming to "abide" in Christ should "walk," or live, as Jesus lived.

This is frequently mentioned as John's ultimate proof for how a person knows he or she is a believer: A believer in Jesus lives like Jesus. True Christians grow to act more and more like Christ. Those who don't demonstrate a Christ-like lifestyle are—at minimum—failing to provide evidence of their salvation. Still, absence of evidence is not always evidence of absence. In other words, failure to grow or change in one's life is not an infallible sign of being lost, nor is a continued struggle with sin. However, anyone who claims to be "in Christ" but fails to live like Him certainly does not have a fellowship relationship with Him.

In chapter 1, John stated that even believers can and do sin. When a Christian sins, they are to confess to God, who will forgive (1 John 1:9). If a person does not live like Christ, and does not confess sin, it gives evidence that the person may not be a true believer. In this letter, John is primarily dealing with believers, and those who falsely claim to be following Christ's commands. Those who fail to follow Christ's will, but say they have "fellowship" with Him, are liars. While it's technically possible for such people to have saving faith, there is no way for them to produce spiritual fruit. The consequences of sin remain, even if we are free from the eternal penalty.