These things we write, so that our joy may be made complete.
There is dispute among scholars as some manuscripts read “our joy” and some “your joy”. A change in the translation would only slightly change the interpretation. Either “our joy” may be complete by bringing the readers also into fellowship with the Father and Son, or “your joy” would be made complete by a clear understanding of who Jesus was and with fellowship with Him and other believers. Strong’s concordance cites the Greek as ἡμῶν (hémón), meaning company, which to me would signify a rendering of “our joy.”
Either way Christ Himself is the source, object, and center of His people's joy, and true joy comes only as a result of the fruit of the Spirit. John was linking his own joy with the spiritual welfare of his readers. He may have had in mind the words of Jesus that he penned in John 15:11, “These things I have spoken to you so that My joy may be in you, and that your joy may be made full.”