John makes the point several times in his gospel that he actually saw the things of which he wrote. At the end of the book he makes several significant statements.
John witnessed the soldiers piercing Jesus' side. He had first-hand confirmation that Jesus was indeed dead. John 19:35 - "And he who has seen has testified, and his testimony is true; and he knows that he is telling the truth, so that you also may believe."
John witnessed the empty tomb. He saw the evidence of Jesus' resurrection. John 20:8 - "So the other disciple who had first come to the tomb then also entered, and he saw and believed."
John witnessed Jesus' first appearance to the disciples after the resurrection (John 20:19). He saw Jesus alive!
Is it any wonder, then, when we begin his first letter (1 John 1:1, 3), we read, "What was from the beginning, what we have heard, what we have seen with our eyes, what we have looked at and touched with our hands, concerning the Word of life...what we have seen and heard we proclaim to you also, so that you too may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ."
It was important that we have eye-witness testimony of all these things. Praise God who has worked everything out so we can have the evidence to believe!
Nathan