“And we are writing these things so that our (your) joy may be complete.” 1 Jn. 1:4
John had heard, seen and touched Jesus. His experience with Him was full and life-changing. Having heard, seen and touched (and been touched by) Jesus, John desired to testify and proclaim to others “the eternal life” made manifest in Him. He wrote this letter that we call 1 John so that others would “have fellowship” and so that “joy (both his and the readers’) may be complete.”
C. S. Lewis said in his “Reflections Upon the Psalms”, “we delight to praise what we enjoy because the praise not merely expresses but completes the enjoyment; it is its appointed consummation.” He also says, “men spontaneously praise whatever they value, [and] so they spontaneously urge us to join them in praising it.” John shares his love and delight in Jesus with others because both his joy and theirs is completed in the sharing.
We should share our love and delight and trust and hope in Jesus with others. This is our “witness”. We do this because we are commanded. We do it because we have compassion for them. We do it so their joy may be complete. But in the sharing, our joy is made complete, too!
To the praise of His glory, and for your joy, testify and proclaim to others the eternal life that has been made manifest to you.