There are many references in Scripture to the effect that God is light
(e.g., Psalm 27:1; Isaiah 60:20; John 8:12). Belonging to God puts you in the light, so that you no longer walk in darkness (without God’s light). See Luke 1:79; Acts 26:18. There is no doubt that these themes play into Peter’s present description of God. The wider context, however, indicates that Peter intends more with this description. Peter had begun this opening section of his letter (i.e., 1 Peter 1:3 – 2:10) with the notion of birth. Pre-birth, life in the womb, is dark; post-birth, life outside the womb, is light. Peter’s readers have been “born again,” from the dark womb of a worldview without God into the light of the worldview that knows God’s redeeming grace in Jesus Christ. Describing God as “him who called you out of darkness into his marvelous light” is to say that God is the cause of their rebirth and at the same time to remind readers of the privileged benefits of this rebirth. The result must necessarily be that Peter’s readers would bless the God and Father of our Lord Jesus Christ (1 Peter 1:3). Doing so is part of proclaiming God’s “excellencies.”
9 But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light.