We proclaim to you what we have seen and heard so that you can have fellowship with us. And our fellowship is with the Father and his son, Jesus Christ. We write this to make your (or 'our' depending on translation) joy complete.
What does it mean to have fellowship with God? To be incorporated into the Trinity? To be called friend (John 15:15) by the Creator King of the universe? What does it mean that we can draw close to the unapproachable One? (1 Timothy 6:16)
More than that: we are in union with Him: Whoever is united with the Lord is one with Him in Spirit. (1 Corinthians 1:16) The word is kollao, to glue, cleave, keep company, to be intimately connected. We are glued to the King. We are intimately bound to Him. And yet this is no mere tacking on. We are not simply pritt-sticked to God or tied loosely to Jesus like some spare part. We are somehow in Him and He is in us. Jesus asked the Father and the Spirit has come to live within us (John 14:16-18). The Eternal God has taken up residence in us. We have become his permanent dwelling place, His home. (John 14:20, 23) Christ in us is the hope of glory (Colossians 1:27). We are hidden inside Him. (Colossians 3:3)
This is madness. We exchange all that we are for all that He is. This is a covenant in which we are the infinitely weaker party; we have nothing to offer and yet we are given everything in return; we are invited in. We are asked to dine with our Maker (Revelation 3:20); we are asked to have fellowship with Him.