If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
We lie - We are false professors; we are deceived if we think that we can have fellowship with God, and yet live in the practice of sin. As God is pure, so must we be, if we would be his friends. This does not mean necessarily that they meant to deceive, but that there was an irreconcilable contradiction between a life of sin and fellowship with God.