If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
and walk in darkness: This situation cannot happen. One cannot live in darkness and light at the same time. "Darkness" often symbolizes ignorance, untruth, and sin. In this instance, the darkness of sin is the primary significance in John’s mind. One cannot live in communion with a holy God and walk in sin at the same time. Sin separates man from his God (Isaiah 59:1-2). God and sin cannot coexist because they are mutually exclusive.