What does 1 John 1:10 mean?

This verse offers the fifth and final conditional ("if") statement in this passage. Sticking to the positive / negative pattern, this one is negative. Verse 8 stated that those who claim to have no sin—presently—are deceived (1 John 1:8). Verse 10 includes prior sins, using the phrase "have not." This verse adds two negative traits of those who claim to be without sin. First, it directly contradicts the word of God, which is the same as calling God a liar. Then, as now, this was considered a great blasphemy.

Second, this claim implies that the truth of the gospel is not in such a person. In other words, a person who claims to be without sin, at present, is not speaking the truth. A person who claims to have never sinned not only speaks a lie, but directly contradicts the gospel. Only Jesus is without sin (Hebrews 4:15). Even most unbelievers acknowledge that they have fallen short of whatever moral code they believe in. Only a person who is deeply deceived would make the claim of being without any sin.

One sect John likely had in mind were the Gnostics, who claimed that all matter was evil and all spirit was good. This led some to believe that sin in the material body was separate from the spiritual person. This is false, as humans are "one" being in multiple parts. Those who make these kinds of excuses for sin are simply living a lie.