What does 1 John 1:8 mean?

This verse offers John's third conditional ("if") statement in a row. There are five such "if" statements in this passage. First, he refers to those who claim fellowship with Christ while living in sin (1 John 1:6). Second, John addresses those who truly live according to Christ's commands (1 John 1:7). Here, in verse 8, John refers to those who claim to have no sin whatsoever.

John's pattern of alternating positive and negative "if" statements clues the reader in to upcoming ideas. Here, it sets up the reader to expect a negative "if" statement. Saying we have no sin is considered negative and simply impossible—only God is completely without sin (Hebrews 4:15). So, anyone who claims to be without sin is self-deceived.

John teaches that those who claim to be without sin do not have the truth in them. This lack of truth applies to the saved believer who claims to have been freed from all sin in their present life. A believer should recognize his or her sinfulness and need of forgiveness through Christ. Even the most devout, clean-living Christian still contends with sin, in some way, shape, or form. Forgetting that we have sin makes us insensitive to things we ought to confess to God. As stated in verse 7, we don't lose hope, but we trust in Christ to forgive us.

While it's not necessary to have perfect knowledge in order to be saved—justified before God—a core aspect of the gospel is a recognition of our sin. Therefore, anyone who claims to have never sinned is denying the gospel. By definition, such a person is an unbeliever. And, any believer who concludes that they no longer sin needs to be corrected.

Note, also, that verses 8, 9, and 10 cover past, present, and future sins. Believers still have the choice of whether or not to submit to Christ at any given time. If this was not possible, the Bible would not warn Christians so often about the consequences of sin. Even the believer is still fallible.