And (kai). Mutual fellowship depends on mutual knowledge (Westcott).
Message (aggelia). Old word (from aggelos, messenger), in N.T. only here and 1Jo 3:11, and note ap' autou (from God like apaggellō in 1Jo 1:3) and anaggellomen, to announce, to disclose, here as in Joh 4:25.
God is light (ho theos phōs estin). Precisely so the Logos is light (Joh 1:4-9) and what Jesus claimed to be (Joh 8:12). John repeats it in negative form as he often does (Joh 1:3).