And these things we write unto you so that your joy may be completely full.1 John 1:4
We come to the purpose statement of this epistle. This purpose statement is glorious in its simplicity, and yet there are deep fathoms here. John has hard things yet to address; he has anti-christs to warn against, he has comfort which needs to be given, he has complex theological/philosophical ideas to explain, and he has sin to rebuke. But he doesn’t front-load any of that. Rather, he reminds his readers that everything which is written to them is intended to make their joy full.
He expands on this purpose statement in the closing sentences of his epistle, and there he says that he has written that they “may know that ye have eternal life, and that ye may believe on the name of the Son of God (1 Jn. 5:13 KJV).” In order to fill their cups brimful with joy, their joy must be tethered to the Eternal Life which comes through faith in the name of the Son of God. Which is the main melody which is recapitulated throughout this letter.
So, John is not interested in just making them circumstantially happy. Rather, he explains that all he is going to write unto them is to the end that they may be in a state of being supremely joyful. He writes these things so that their joy might be complete, overflowing, spilling over, and with a cherry on top.
The Greek word which I translate here as “completely full” is πεπληρωμένη; another time this exact inflection of the word is used is when Jesus tells the disciples to “ask, and ye shall receive, that your joy may be full (Jn. 16:24).” There the joy is full by asking of the Father in Jesus name. Here our joy is full by receiving the words which John proclaims. Putting those two passages together we see that the believer’s joy is made full by praying to the Father in Jesus and hearing from the Father of Jesus.