Darkness means not knowing God and being controlled by sin (see 4:18).
Light in the Lord is the good news of Christ (see John 8:12). As people of light, believers live in fellowship with God, who has no darkness (see 1 John 1:5–7; 5:9; compare Matthew 5:14–16; John 8:12; 12:35–36, 46; Romans 13:12; 2 Corinthians 6:14; 1 Thessalonians 5:4–8; 1 Peter 2:9).