If we claim that we experience a shared life with him and continue to stumble around in the dark, we're obviously lying through our teeth—we're not living what we claim. But if we walk in the light, God himself being the light, we also experience a shared life with one another, as the sacrificed blood of Jesus, God's Son, purges all our sin.
If we claim that we're free of sin, we're only fooling ourselves. A claim like that is errant nonsense. On the other hand, if we admit our sins—make a clean breast of them—he won't let us down; he'll be true to himself. He'll forgive our sins and purge us of all wrongdoing. If we claim that we've never sinned, we out-and-out contradict God—make a liar out of him. A claim like that only shows off our ignorance of God.