This is the message we have heard from him and declare to you: God is light, and there is absolutely no darkness in him.
Here's why I think this passage is so surprising. Ask any Christian about the content of the gospel, the message at the heart of the Good News, and I expect no one responds with "God is light." And yet, that's how 1 John summarizes the content of Jesus' message ("the message we heard from him"): "God is light."
Jesus comes to us in the Incarnation with a message, to tell us that God is light.
That's crazy, mystical stuff.
And I'm not just cherry picking a single verse. This contrast between light and darkness is one of the main images in the Johannine texts. Some selective passages from the Gospel of John:
In him was life, and that life was the light of men. That light shines in the darkness, and yet the darkness did not overcome it.
"This is the judgment: The light has come into the world, and people loved darkness rather than the light because their deeds were evil."
“I am the light of the world. Anyone who follows me will never walk in the darkness but will have the light of life.”
"I have come as light into the world, so that everyone who believes in me would not remain in darkness."
As I noted above, 1 John 1:5 is a slight modification of this theme. In the gospel of John, Jesus is the light. In 1 John we read that Jesus' message about God was that "God is light."
What is the import of the message that "God is light"? I'll confess: I think about "God is light" a lot. In contemplative prayer, I mainly see and experience God as light. I don't think the primary meaning here is moral, though there are moral implications. I think the meaning of "God is light" is primarily metaphysical and concerns the nature of God.
Of course, we need to avoid literalism here. As the mystical tradition claims, God is the "Uncreated Light." So our experience of created light is only analogically related to God. As the psalmist says of God, "In your Light we see light."
But again, what does this mean? We just have, in our experience of light, a cloud of associations. Brilliance. Radiance. Illumination. Luminosity. Clarity. But what might any of these associations mean in relation to God's nature and being? That is a question I ponder a lot.
Regardless, 1 John 1:5 is a startling, mystical claim. Jesus' message was "God is light."