What does 1 John 5:20 mean?
Jesus has provided two important things for the believer. First, Jesus has come (1 John 4:2). John has noted the importance of accepting Jesus as coming in the flesh. Christ's life and death as a real, flesh-and-blood man is a crucial aspect of the gospel. By experiencing suffering as a human being, Jesus could identify with our struggles (Hebrews 4:15). He could also fulfill the role of the Promised One (John 1:29–30).Second, Jesus gives believers understanding. Jesus did this for two reasons. First, so we can be saved. Second, to give us fellowship with God. The phrase "his Son Jesus Christ" is used only here and in 1 John 1:3, making it a fitting bookend to his letter. First John 1:3 notes, "our fellowship is with the Father and with his Son Jesus Christ."
John's reference to truth is especially important. Part of John's letter has dealt with the problem of false teachers (1 John 2:26). Paul's letters referred to those who preached a different gospel (Galatians 1:8). Faith in Christ gives believers spiritual discernment (1 Corinthians 2:14), making it easier to distinguish between true and false teachers (1 John 4:6).