Each has his own role in the Kingdom of God. Peter had his and John had his. The previous verse indicates that John lived to be very old, which allies itself with tradition. His function was to be the witness to Christ. His entire gospel is an affidavit to Who Jesus was, what He said, and what He did. Who better to write a gospel than the disciple closest to Christ? John has faithfully recorded testimony from various sources throughout his book. He has used the word ‘witness’ as a noun 14 times and as a verb 33 times. These words are found a total of 4 and 2 times respectively in the other three gospels thus stressing the role God had for John in writing this gospel. Among those bearing witness to Jesus in the Gospel of John are:

1. the Father (5:37, 8:18)
   John 5:37  And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form,
   
   John 8:18  I am one who testifies for myself; my other witness is the Father, who sent me.”
2. the Spirit (15:26, 16:14)
   John 15:26  “When the Counsellor comes, whom I will send to you from the Father, the Spirit of truth who goes out from the Father, he will testify about me.
   
   John 16:14  He will bring glory to me by taking from what is mine and making it known to you.
3. Jesus Himself (8:14, 18, 3:11, 32, 8:37)
   John 8:14  Jesus answered, “Even if I testify on my own behalf, my testimony is valid, for I know where I came from and where I am going. But you have no idea where I come from or where I am going.
   
   John 8:18  I am one who testifies for myself; my other witness is the Father, who sent me.”
   
   John 3:11  I tell you the truth, we speak of what we know, and we testify to what we have seen, but still you people do not accept our testimony.
   
   John 3:32  He testifies to what he has seen and heard, but no-one accepts his testimony.
   
   John 8:37  I know you are Abraham’s descendants. Yet you are ready to kill me, because you have no room for my word.
4. the works of Christ (5:36, 10:25, 14:11, 15:24)
   John 5:36  “I have testimony weightier than that of John. For the very work that the Father has given me to finish, and which I am doing, testifies that the Father has sent me.
   
   John 10:25  Jesus answered, “I did tell you, but you do not believe. The miracles I do in my Father’s name speak for me,
   
   John 14:11  Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the miracles themselves.
   
   John 15:24  If I had not done among them what no-one else did, they would not be guilty of sin. But now they have seen these miracles, and yet they have hated both me and my Father.
5. scripture (5:39, 5:45-47)
   John 5:39  You diligently study the Scriptures because you think that by them you possess eternal life. These are the Scriptures that testify about me,
   
   John 5:45-47  “But do not think I will accuse you before the Father. Your accuser is Moses, on whom your hopes are set.  If you believed Moses, you would believe me, for he wrote about me.  But since you do not believe what he wrote, how are you going to believe what I say?”
6. John the Baptist (1:6-8)
   John 1:6-8  There came a man who was sent from God; his name was John.   He came as a witness to testify concerning that light, so that through him all men might believe.  He himself was not the light; he came only as a witness to the light.
7. the disciples (15:27)
   John 15:27  And you also must testify, for you have been with me from the beginning.
8. the Samaritan woman (4:39)
   John 4:39  Many of the Samaritans from that town believed in him because of the woman’s testimony, “He told me everything I ever did.”
9. the multitude (12:17)
   John 12:17  Now the crowd that was with him when he called Lazarus from the tomb and raised him from the dead continued to spread the word.
10. and John himself in this verse and in (19:35).
    John 19:35  The man who saw it has given testimony, and his testimony is true. He knows that he tells the truth, and he testifies so that you also may believe.

Your individual goal in serving Jesus should be to find what role or roles He has for you and then do them in total dependence upon Him and His abilities and not upon your own (Php 4:13).

Philippians 4:13  I can do everything through him who gives me strength.