"God is light." (1 John 1:5)
The biblical text is rich with metaphors and similes, one of which often appears in John's writings. God is said to be "light"--the most constant, clearly observable, and all-pervasive experience in our universe.
God's life is the light of men (John 1:4).
God's light is not conquered by darkness (John 1:5).
God's light attracts men who love truth (John 3:21).
Jesus is the "light of the world" (John 8:12).
John's emphasis in his epistle is focused on the application of the "light" in our lives. Since God IS light (our text; see also 1 Timothy 6:16), we can never be a participant in the life of God apart from the light of God (1 John 1:6). If we claim fellowship with God, we must "walk in the light, as he is in the light" (1 John 1:7). Since God is the "true light" (1 John 2:8), we are not part of His family if we despise those He loves (1 John 2:9).
It is equally obvious that since God is holy (Psalm 99:9) and righteous (Daniel 9:14), the light that we are to "shine" (Matthew 5:16) must be a "radiant" righteousness that is visible to all who come in contact with us (Proverbs 4:18; 1 Thessalonians 5:5).
Our breastplate of righteousness (Ephesians 6:14) should "blind" the ungodly with the brilliance of our lifestyle of holiness--so much so that even if we are spoken against by those who hate God, they will be forced to glorify God ("adorn with luster") because of our good works (1 Peter 2:12).
Because the God of our salvation is "the light of the world" (John 9:5) and we have been made "the children of light" (Ephesians 5:8), "ye should shew forth the praises of him who hath called you out of darkness into his marvelous light" (1 Peter 2:9). HMM III