"This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say we have not sinned, we make him a liar, and his word is not in us." - 1 John 1:5-10
You're right, the Christian does not walk in darkness because we have been illuminated with the truth of the Gospel, as St. Paul says,
"giving thanks to the Father, who has qualified you to share in the inheritance of the saints in light. He has delivered us from the domain of darkness and transferred us to the kingdom of his beloved Son, in whom we have redemption, the forgiveness of sins." - Colossians 1:12-14
"He has qualified you", how? By justifying us freely by His grace.
And so being justified He has caused us to share in the inheritance of the saints in light.
So who is the one who walks in the light? The one who is justified. The one who is justified has been made a saint, not by a holiness which they themselves posses, but by the holiness of Jesus Christ. And so having been justified, redeemed, saved, we have been delivered from the domain of darkness, having been liberated from the captivity of the devil, recall the author of Hebrews who wrote,
"Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil, and deliver all those who through fear of death were subject to lifelong slavery." - Hebrews 2:14-15
For Christ having defeated the devil, destroying the power of death, has brought freedom to the captive. And so we, who were captives of the devil, held in bondage to sin, held in slavery to death, have been redeemed and ransomed, "You are not your own, you were bought with a price." (1 Corinthians 6:19-20). So having been ransomed, redeemed, purchased by the precious blood of the Lamb of God we have been delivered from darkness into the light of God, the light of God's only-begotten Son, who has made us free, as it is written, "If the Son has set you free, you are free indeed." (John 8:36).
And so we are now called to walk in the light, even as He is in the light. For the blood of Christ has cleansed us of all our sins.
But, says the Presbyter, if we say we are without sin then we are liars. For how can the sinner, purchased by Christ's blood, now turn and say they are without sin? For it is precisely their sin that condemned them. So how can the one who has been justified turn around and say "I have no sin." So having said this such a person has denied the very Gospel which has given him his freedom. Instead the one who is justified and redeemed must be reminded of this precious Gospel, that if we confess our sins God is faithful and just to forgive us our sins and cleanse us from all unrighteousness. The one who has believed upon the Gospel is one who falls down and cries like the righteous publican, "Lord, have mercy on me a sinner." So the one who says they have not sinned, the one who says they are not a sinner, this is one who has not believed the Gospel, if we tread against the truth like this, His word is not in us, we deny the Gospel and dwell in falsehood.
It is the redeemed man who seeks after God, not the unregenerate; for the unregenerate cannot, as it is written, "no one seeks after God." (Romans 3:11).
If the Spirit is alive in us, then we fall to our knees in repentance, hoping and trusting in the grace of God, for it is only by the Spirit that we have faith, it is only by the Spirit that one can say, "Jesus Christ is Lord." and we know that whoever calls upon the name of the Lord shall be saved, and shall never be put to shame, as it is written, "For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, 'Everyone who believes in him will not be put to shame.' ... For 'everyone who calls on the name of the Lord will be saved.'" (Romans 10:10-11, 13) And this word has gone forth, preached and declared, and by this precious Gospel we have received faith, as the Apostle writes, "For faith comes by hearing and hearing by the word of Christ." (Romans 10:17)
Therefore pray and pray hard, for you are quite a sinner.
And so am I.
Lord Jesus Christ, Son of God, have mercy on me, a sinner.
-CryptoLutheran