If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
While most would scoff at such a notion, there are those today who claim that once a person is saved there is no way he can be lost. The sins committed are not held against him despite his impenitence. Is this doctrine any less damaging than that of the Gnostics? John declares that fellowship with God negates a walk in sin.