We know what walk means. It implies how we conduct our lives. I John is a letter written to a church congregation, and he is instructing church members that fellowship hinges on walking in the light. This is how we are to have fellowship with one another.
The context of this instruction is quite interesting. Protestantism focuses heavily on the initial forgiveness of sin that takes place upon belief at the beginning of salvation. Thus, its evangelists have altar calls. People come down before the altar and confess their sins, accepting Christ as their Savior. Once they do this, these people are considered "saved" and "born again." Their doctrine—that of eternal salvation—continues in this vein; that is, once that occurs, salvation is basically assured. So, it places heavy emphasis on initial repentance and forgiveness of sin.
However, notice this verse in its context. John writes, "If we walk in the light. . . ." An individual cannot walk in the light until he is called and converted. This walking occurs after conversion. He continues, "If we walk in the light, as He is in the light, we have fellowship . . .." The fellowship depends on what we do after initial repentance. Amos writes, "Can two walk together, unless they are agreed?" (Amos 3:3). It cannot be done. We do not have fellowship with people that we do not agree with. Agreement is shown by the way that we conduct our lives—how we behave under our belief system.
In context, the phrase ". . . and the blood of Jesus Christ His Son cleanses us from all unrighteousness" is the game-breaker. The apostle is teaching that further cleansing, further forgiveness, hinges on our obedience to God after we are converted, for "walking in the light" is synonymous with obedience or living righteously. Forgiveness after conversion works the same way as the forgiveness God gave us before we were converted. It hinged on whether we had repented and had begun obeying God.
Clearly, salvation is a process!
What we must understand here is that forgiveness, cleansing, and even fellowship are not once-for-all acts. They are part of a process—even as growing in the grace and knowledge is a process, even as the writing of God's law on our heart is a process. Cleansing is a process. The quality of the fellowship depends upon all of these things. So, if we walk in the light, we have fellowship, and His blood cleanses us.

