John is traditionally identified with the Beloved Disciple, on whose testimony the fourth Gospel is based (Jn 21:24). John testified that he surely could not help but speak of what he had heard and seen (Acts 4:19-20). Everything within John found joy in sharing his experience of Jesus' love with others (1 Jn 1:3-4).
John saw Jesus. He heard Jesus' heart beating (Jn 13:25). John stood at the foot of the cross and watched Jesus pour out His blood to save us (Jn 19:26). He was in a position to testify that "the Word became flesh" (Jn 1:14).
Even with all this experience of Jesus' physical, human presence, John's love for Jesus longed to go deeper. John, "the one Jesus loved" (Jn 20:2), experienced Jesus' love so deeply that he "was caught up in ecstasy" (Rv 1:10) into the mystery of love. He encountered the divinity of Christ "in heaven" (Rv 4:2). Because of this encounter (see Rv 5:6ff), John knew that Jesus was God (Jn 1:1). His Gospel, more than any other, proclaims the divinity of Jesus. John was consumed with the desire to proclaim that the Baby Jesus in the lowly Christmas manger is the divine God enthroned in heavenly majesty.
John wrote "to help you believe that Jesus is the Messiah, the Son of God, so that through this faith you may have life in His name" (Jn 20:31). What John saw and heard, he proclaimed "in turn to you so that you may share life" with him and Jesus forever (1 Jn 1:3). Look, hear, believe in Jesus, and live forever.