He who loves his brother remains in the light, and there is no occasion for stumbling in him.
Abideth in the light ...  There can hardly be any doubt that John had in mind the great declaration of Jesus Christ that "I am the light of the world: he that followeth me shall not walk in darkness" (John 8:12), and that this means abiding in Christ, a thought recurring repeatedly in  John 15:1-10.