A.T. Robertson Commentary 1 John 1:5

"And this is the message which we have heard from him and announce unto you, that God is light, and in him is no darkness at all." — 1 John 1:5 (ASV)

And (κα). Mutual fellowship depends on mutual knowledge (Westcott).

Message (αγγελια). Old word (from αγγελος, messenger), in N.T. only here and 3:11, and note απ' αυτου (from God like απαγγελλω in verse 3) and αναγγελλομεν, to announce, to disclose, here as in Joh 4:25.

God is light (ο θεος φως εστιν). Precisely so the Λογος is light (John 1:4–9) and what Jesus claimed to be (John 8:12). John repeats it in negative form as he often does (John 1:3).