“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.”

1 John 1:7 “And the blood of Jesus his Son cleanseth us from all sin kai (NT:2498) to (NT:3543) haima (NT:127) Ieesou (NT:2389) tou (NT:3543) huiou (NT:5148) autou (NT:839) katharizei (NT:2477) heemas (NT:2214) apo (NT:569) pasees (NT:3909) hamartias (NT:263). This clause with kai (NT:2498) in true Johannine style is coordinate with the preceding one. Walking in the light with God makes possible fellowship with one another and is made possible also by the blood of Jesus (real blood and no mere phantom, atoning blood of the sinless Son of God for our sins). John is not ashamed to use this word. It is not the mere "example" of Jesus that "cleanses" us from sin. It does cleanse the conscience and life and nothing else does (Hebrews 9:13 f; Titus 2:14). See in 1 John 1:9 both forgiveness and cleansing. Compare 1 John 3:3. (from Robertson's Word Pictures in the New Testament)

1 John 1:7 Cleanseth katharizei (NT:2477). See the note at Mark 7:19. Not only forgives but removes. Compare Titus 2:14; Hebrews 9:13 ff; Hebrews 9:22 ff; Ephesians 5:26 ff; Matthew 5:8; 1 John 3:3. Compare also 1 John 1:9, where forgive afee (NT:858) and cleanse katharisee (NT:2477) occur, with an obvious difference of meaning. Note the present tense cleanseth. The cleansing is present and continuous. (from Vincent's Word Studies of the New Testament)

1 John 1:7 [The blood of Jesus ... cleanseth us from all sin]-daily contracted through the weakness of the flesh, and the power of Satan and the world. He is speaking, not of justification through His blood once for all, but of present sanctification ("cleanseth"), which the believer, walking in the light, and having fellowship with God and the saints, enjoys as his privilege. Compare John 13:10, `He that has been bathed [leloumenos (NT:3026)] needeth not save to wash [nipsasthai (NT:3494)] his feet, but is clean every whit.' Compare 1 John 1:9, "cleanse us from all unrighteousness:" a further step besides `forgiving us our sins.' Christ's blood is the mean whereby, being already justified, and in fellowship with God, we become gradually clean from all sin which would mar fellowship with God. Faith applies the cleansing, purifying blood.  (from Jamieson, Fausset, and Brown Commentary)

1 John 1:7 heemas (NT:2214), continues to cleanse us, i.e., to keep clean what it has made clean” (from Adam Clarke's Commentary)

“All sin” pasees means sin in all its forms and activities. John refutes the error that we have no sin nature, by using the word sin in the singular. It is not just the committal of sin but the principle of sin, sin in a general sense, sin of every description. The basic definition of sin is the transgression of the law. The Greek word for sin means “missing the mark,” God’s law, and His commands which come from his nature are the mark or standard that we aim at and miss. 1 Jn.5:17 “All unrighteousness is sin,” and anything not done in faith is sin.

God's means of cleansing the sinner is by the blood of Jesus, shed on the cross, which cleanses us completely and brought us into fellowship with God; this same blood of Jesus keeps us clean from all sin that would sever that fellowship. This is a continual cleansing from daily sins that are present from our fallen nature. The believer still struggles against his sinful nature, the confession of sin brings our trespasses to the light. The Scripture tells us To “walk in the light, as he is in the light” to have fellowship (koinonia) with the brethren and God.

This means lives of holiness and purity; characteristic of being a light in the world, an example of Christ. We are to walk in the truth; that is, embracing the truth in opposition to error. John 3:19- those who are darkness like it, and do not want to come to the light. God is light and Christianity is about those who come to light to know the depth of their darkness.