1 John 1:7

but if we walk in the light, as He is in the light, we have fellowship one with another, and the blood of Jesus Christ, His Son, cleanseth us from all sin.

The apostle now launches forth in his letter proper, developing, first of all, his topic that God is Light: And this is the message which we have heard from Him and announce to you, that God is Light and that darkness is in Him in no way. St. John desires to make an announcement, a declaration, to deliver a message. It is not a message or philosophy which he has thought out himself; he is not offering the result of any research of his own. What he writes, what he proclaims, is the truth of Christ, of God; he is a messenger of Christ, a minister of salvation, as every true pastor is intended to be. God is Light, and darkness in Him there is none. Light is purity, holiness; He is the Source of all true knowledge, wisdom, happiness, and holiness. There is no darkness, no ignorance, no imperfection, no misery, no sinfulness in Him. As light is the symbol of purity, goodness, and perfection, so, on the other hand, darkness symbolizes ignorance, sinfulness, misery, corruption.

Upon this fact the apostle bases a conclusion regarding the conduct and life of the Christians: If we say that we have fellowship with Him and walk in darkness, we are liars and are not practicing the truth. That we have fellowship with God as our heavenly Father by faith the apostle had just stated. But if we now, who profess to be Christians and thus to be united with God in the most intimate union, live and behave ourselves as though we were still in darkness, if we are addicted to sin, if we in any way serve sin and corruption, then our entire life is a lie. We may be self-deceived, under circumstances, but the lie is there nevertheless. We are then not doing, practicing, the truth, which demands that we live a pure and holy life, according to the will of our heavenly Father. To walk and live in sins while professing to be children of God is to brand ourselves as liars and hypocrites.

St. John describes the conduct of the Christians as it should be: But if we walk in the light as Himself is in the light, we have fellowship with one another, and the blood of Jesus, His Son, cleanses us from all sin. We are a light in the Lord through faith, and therefore it behooves us to walk as the children of light, Ephesians 5:8. God, our heavenly Father, is in the light, His entire essence is holiness, everything that He does is pure and holy. Of this nature we partake by faith, and our conduct should give evidence of the faith that has made us children of the light and enables us to walk as the children of light, according to God's good pleasure and will. If thus we live a holy and righteous life, deriving continual light, power, and life from Him, then there will be two happy consequences of such behavior. In the first place, we have the assurance that we have fellowship with one another: we are closely connected with our heavenly Father by faith; we are united with the holy apostles and with the Christians of all times by the bond of this same faith. Just as an unholy, sinful life, a conduct of sin and shame, excludes the perpetrator from all communion with the saints of God and with God Himself, so a righteous and holy life, lived by the power of God through faith, binds us ever more closely to the Lord and to one another. At the same time we are also assured that the blood of Jesus, our Savior, the Son of God, cleanses us from all sin. In spite of the weaknesses and imperfections of this earthly life, in spite of the many accusations and temptations on the part of the devil and the children of this world, we have forgiveness of sins. Jesus, the true Man, our Brother according to the flesh, but at the same time the Son of God, the eternal God Himself, has shed His blood for us once, yet His sacrifice has eternal validity and power by virtue of that mysterious, wonderful personal union of the two natures. Always, every day, without ceasing, we have forgiveness of sins, we are righteous and just and holy before God through the blood of Jesus Christ, which is always effective; in the case of every sin we have forgiveness, which is always and ever again offered and transmitted to us in the Word and in the Sacrament and accepted by us in faith.