If we say that we haven't sinned, we make him a liar, and his word is not in us.
and his word is not in us: This statement is equivalent to the one in verse 8, "the truth is not in us." God’s word is truth (John 17:17). To have God’s word in us is to have His truth in us. The person who denies the presence or practice of sin in his life testifies in that denial to the absence of God’s word or truth in his heart. Paul commands, "Let the word of Christ dwell in you richly..." (Colossians 3:16). When the word is genuinely in us, we believe and apply its teachings. The denial of sin in our lives is proof that the word has not been allowed to take up its permanent residence in our hearts.