“But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin.”
Cleansing from sin is a prerequisite for fellowship with God. The blood of Christ continually cleanses the believer from sin (present tense). To the degree that we live in the light of who God is, we can fellowship with Him. This is no fresh removal of sin but that Jesus removes our sin on the ground of His expiatory death on the cross. Both justification and sanctification rest on the blood of Christ.
We get our English word “cathartic” from the Greek word for “cleanses.” The blood of Christ cleanses us from sin on a temporal, daily basis.
from all sin
Note that the blood of Christ cleanses us from “all sin.” God cleanses all sin, not some sin. The idea is that the blood of Christ cleanses us from any and every sin that we might commit. There is no sin beyond the cleansing power of the cross. He cleanses us from sins committed consciously or unconsciously.
Christ can cleanse us from any sin that might occur. God has just claims against us because of our personal sin but Jesus’ blood satisfies the holy demands of God. The death of Christ on the cross fully saves sinners.
Cleansing from sin is a prerequisite for fellowship with God.
In addition to positional sanctification, believers need a progressive sanctification. God sanctifies us positionally one time at the point of salvation 1 Co 6:11). After salvation, we need to continually grow in holiness. That is the progressive cleansing of 1:7 and 1:9.
Just as light and darkness cannot co-exist with one another, so sin and God cannot co-exist together. God is absolute perfection and cannot co-exist with sin in any sense. A believer with active, unconfessed sin in his life cannot walk or fellowship with God. He cannot lose his standing with God but he can lose his fellowship with God.
Fellowship with God as a Christian depends on confession of sin. A carnal Christian cannot fellowship with God. However, God cleanses the carnal Christian if he confesses his sin. There is a cleansing after salvation (1:9). The basis of forgiveness after we become a Christian is the same as when we became Christians.
When we become Christians, we establish an eternal relationship with God. However, at the moment sin comes into our lives, we break fellowship with God. Sin puts a cloud between us. There is something between the soul and the Savior. Your son is still your son even though he may have shamed the family. He is still in the family although there is a strain on your fellowship.
Walking in the light means that we hide nothing from God. We do not rationalize our sin away but admit it and deal with it.