But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus Christ, his Son, cleanses us from all sin.
This great verse is the source of incredible joy, assurance and consolation to the child of God. He never needs to fear that some impulsive, unintentional, or atypical conduct might overtake him with the result of eternal condemnation.  His walking "in the light" can be established by the long term directional thrust of his whole life upon earth and cannot be contradicted and negated by any temporary or insignificant lapse.