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.
Now this verse has its application to the problem of fellowship because the false teachers were walking in a moral darkness of the worst intensity, while at the same time claiming to be "in God."  The impossibility of their pretensions having any merit was proved by this very first sentence of the message proper.  It is preposterous for one to claim fellowship with God while walking in darkness.