New Thoughts
Well, there seem to be a few points of disagreement here: 1) whether John speaks of Jesus as eternal Word, or temporal manifestation, or both. I still tend to see the 'from the beginning' reference pointing to the larger picture of the eternal Word - as it does in John's gospel. (2) whether John expects familiarity with his gospel as prerequisite to what's said here. Rather a moot point. Doesn't appear to change the meaning of anything. (3) whether it's John's joy or ours that is to be completed. This is an argument between manuscripts. Either meaning is significant, and perhaps looking at it as both is the best solution. Both his joy and ours will be completed in fellowship.
This is the larger answer to our question. The fellowship John offers and seeks - with God and with the Church - as it unites us in common purpose with God, and joins us to the work of Christ, allows us to fulfill God's commandments and thus find our joy complete. It allows us to work in the role of the bridegroom's friend - uniting the bride with the groom, thus also completing our joy. It allows us the reconciliation to God offered in the Holy Spirit through Christ's atoning work, thus perfecting our joy.
Oh, to be so closely united with my Lord. To understand His deepest feelings, and share them as my own. God, I have so far to go to reach that point. I've not yet even managed it with the family I share this life with, how then shall I manage it with You? You've taught that we cannot love the invisible God if we cannot love our visible neighbors, and yet without Your abiding presence, we cannot even pretend to that love. God, work this clay. Soften it as You must to change it. Where I've become hardened, break me, Lord. Grind me to dust that you may return me to a malleable material with which to work. Lord, I trust to Your gentle hands this frail spirit; knowing You have my best in mind, knowing that whatever You choose to do, however much it may hurt, You are doing it to bring me to a better place. Oh, Father, I don't expect to ever be perfect in this life, but I do long so for it - to be freed from the weaknesses that plague, to be freed from the callousness that this harsh life brings to my nature. Do Your work in and through me, Lord, that I may not be found wanting in the last days, but may be called Onesimus. Change my heart, make me whole, burn out the dross, that my life may shine with Your presence in me. Amen.