Separated from his readers by distance, John sees a threat to their faith and to the joy they have in Christ. He decides to write to them, in order to warn them about the danger of another Jesus and help them continue firm in the truth.
Thus we are writing these things so that our joy may be complete.
1 John 1:4 NET
The joy belongs to John and his readers. His letter is illuminating, informative, and confirming. More than that, the reading of the letter effected a transformation in them. The writing, as an oracle of God, is also an act that produces change.
Separated from the author by time, we still experience, in our reading of the letter, the same effect of complete joy by being in fellowship with the Father, the Son, and the brotherhood that confesses that Jesus came in the flesh and by being certain of our salvation through obedience to God’s commandments.