If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
The Greek word translated "fellowship" (koinonia) here means sharing by two or more parties. It does not refer to sharing salvation. Some commentators take the phrases "have fellowship with Him" and "walk in the light" as describing salvation. [Note: E.g., Lloyd-Jones, pp. 130, 142. ] Advocates of this view say that if a Christian does not persevere in the faith he or she is not a Christian. This interpretation may result in back loading the gospel with works. One writer held that "walking in the light" describes the criteria for access to the Father. The criteria for that validation is not good works but believing the revelation of imputed righteousness and forgiveness of sins through Jesus Christ, which John defines as "the light" (revelation). Thus, one "walks in the light" if he or she believes in Jesus Christ. If one "walks in darkness," he or she does not believe in Jesus Christ and thus has no access to the Father. [Note: Charles P. Baylis, "The Meaning of Walking ’in the Darkness’ (1 John 1:6)," Bibliotheca Sacra 149:594 (April-June 1992):214-22.] 