Read 1 John 1:5-10

But if we are living in the light, as God is in the light, then we have fellowship with each other, and the blood of Jesus, his Son, cleanses us from all sin. - 1 John 1:7 (NLT)

Sin lives in darkness; when we are in darkness, we can sometimes even convince ourselves that God doesn’t know what we’re doing (as if anything could be hidden from God). John implores us to deliberately live our lives in the light of God, so that our sin can be revealed to us as sin, and can be confessed, and can be forgiven by God for the sake of our Savior Jesus Christ.