John's words point to the vivid recollection of the person of Jesus that John still had in his old age. For John those memories were permanently etched on his mind as if the events had just happened. He uses terms such as "we have heard" "we have seen" "we have looked upon" and "our hands have handled" that strongly affirm the physical reality of the incarnation of Jesus.
John's personal testimony refutes the false teachers who boasted arrogantly and wrongly about the Christ they had never seen or known. John wanted his readers to be partakers with him in possessing eternal life. We can have fellowship with the Father and with His Son Jesus Christ, which ultimately produces a fullness of joy.
John sums up all he had learned from Jesus in the message: " ... that God is LIGHT, and in Him is no darkness AT ALL." Light in the Scriptures is the emblem of perfect purity, truth, knowledge, prosperity, and joy. In Him, there is NO darkness at all, not even a pin-prick. Where there is pure light, there is pure love. On the other hand, darkness is the absence of light. When people stray from believing in God and having love for others, they step back into the darkness and act in ways that are inconsistent with His character. Darkness represents what is sinful and evil. God is untainted by any evil or sin. Because "God is light" He alone can guide people out of the darkness of sin and unbelief.