This is the message which we have heard from him and announce to you, that God is light, and in him is no darkness at all.
In him is no darkness at all. This mode of speaking is commonly used by John, to amplify what he has affirmed by a contrary negation. Then, the meaning is, that God is such a light, that no darkness belongs to him. It hence follows, that he hates an evil conscience, pollution, and wickedness, and everything that pertains to darkness. 