Symbolism, Imagery, Allegory
We never actually see Milton's God; the only real thing Milton says about him is that he's really bright, or that he's like a giant light, hidden away somewhere. One of the ways in which Milton indicates a particular character's virtue is by how "bright" they are. So, for example, Satan refers to the "bright confines" (2.395) of Heaven, and Milton notes that "God is light,/ And never but in unapproachèd light/ Dwelt" (3.305). Other references to God's "glorious brightness" (2.376) are scattered throughout the poem (such as 3.375).
The angels are also always described as bright, luminous beings. For example, in Book 3 Milton says they "Stood thick as stars" (61), stars being some of the brightest objects in the universe (the sun is a star!). In Book 5, Adam sees Raphael approaching from the east (where the sun rises) and even thinks it's a second sunrise ["seems another morn/ Risen on mid-noon," (5.309-311)].
Satan provides a nice contrast. When he lived in Heaven, he was "Clothed with transcendent brightness" (1.86) and "didst out-shine/ Myriads" (1.86-87). Later we learn that, after he fell from Heaven, Satan "his form had yet not lost/ All her original brightness" (1.591-592), a remark that suggests he has lost at least some as a result of his sin. Our suspicions are confirmed by Raphael, who notes that he was "brighter once" (7.132). A similar description occurs at 10.445.