In this passage John speaks of a believer enjoying his acceptance as a child of God Fellowship with God brings the joy spoken of in this passage. There is joy in knowing that I am accepted by God, not on the basis of my behavior, but on the basis of my birth! It should be noted that being "born of God" is mentioned no less than seven times in this book.
What's the main point? You, as a child born of God, are as welcome to come into His presence as if you were Jesus Christ, Himself! You're not just a member of a church! You're part of God's family! That's more than just a "name on a roll!" You are as close to God as your child is as close to you! Your child is part of your life. You can't imagine life without your children! And God treats us the same way!
When John speaks of "our fellowship is with the Father and with His Son, Jesus Christ" it might also read, "our fellowship with the Father is a close as Jesus is to His Father!" The words, "and with His Son" can also be read, "even as His Son." And the tragic thing is that we haven't taught this as John would teach it!
We've been taught to focus on our behavior as a Christian at the expense of hardly thinking of our birth as a child of God. We've been taught to think that we can't get close to God unless we "straighten out our ways and live as we should." And what's the consequence of that? We tend to shy away from spending time with God because we think He won't ever approve of us because we're not living as we should. We ignore Him because we think He is constantly annoyed with us!
We keep trying to correct our behavior when we need to focus on our birth! I am a child of God because I am born again into His family. And I am always accepted by Him on that basis!