God Appears & God is Light To those poor Souls who dwell in the Night, But does a Human Form Display To those who Dwell in Realms of day. (129-132)
When Blake says that God appears as "Light" to "poor Souls" who dwell in "Night," he's saying that God doesn't typically appear to human beings, what with their living in the material world. The closest thing we have is light from the sun—since it's this warming force that keeps everything alive.
Even though the sun's pretty amazing, it's just a pale reflection of the power of God. Plus, light is inhuman—it's just an impersonal, natural force. But, says Blake, when you enter the eternal world, God doesn't appear as a blob of light anymore—instead, he's a "human," in the sense that he's this Cosmic Human or "Human Form Divine," which Blake considers to be the same as Christ. God appears as the being he really is—someone who isn't less than human, and is infinitely capable of loving and caring for everything that lives.