The Gospel of John is the only Gospel which mentions “the disciple whom Jesus loved.” John 13:23 tells us, “One of them, the disciple whom Jesus loved, was reclining next to Him.” John 19:26 declares, “When Jesus saw His mother there, and the disciple whom He loved standing nearby, He said to His mother, ‘Dear woman, here is your son.'” John 21:7 says, “Then the disciple whom Jesus loved said to Peter, ‘It is the Lord!’” This disciple is never specifically identified, but the identity of the disciple whom Jesus loved is clear. The disciple whom Jesus loved is John, the son of Zebedee and brother of James.
First, only the Gospel of John mentions the “disciple whom Jesus loved.”
Second, John 21:2 lets us know who was fishing with Peter: “Simon Peter, Thomas (called Didymus), Nathanael from Cana in Galilee, the sons of Zebedee, and two other disciples were together…” The apostle John was a son of Zebedee (Matthew 4:21).
Third, there were three disciples who were especially close to Jesus: Peter, James, and John (Matthew 17:1; Mark 5:37; 14:33; Luke 8:51). The “disciple whom Jesus loved” could not be Peter, as Peter asks Jesus a question in regards to this disciple (John 21:20-21). That leaves us with James or John. Jesus made a statement about the possible “longevity” of the life of the disciple whom He loved in John 21:22. James was the first of the apostles to die (Acts 12:2). While Jesus did not promise the disciple whom He loved long life, it would be highly unusual for Jesus to say, “If I want him to remain alive until I return, what is that to you?” if the disciple whom He loved was going to be the first disciple to die.
Church history tells us that the apostle John lived into the A.D. 90s and was the last surviving apostle. Early church tradition was unanimous in identifying John as the disciple whom Jesus loved. It seems that John had a closer relationship with Jesus than any of the other disciples. Jesus and John were essentially “best friends.” Jesus entrusted John with;
The care of His mother Mary,
The vision of the transfiguration
Being an eye witness of His most amazing miracles
Being taken to Heaven bodily to be given the extraordinary visions he later recorded as the Book of Revelation
(Adapted from “Who was the disciple that Jesus loved?” https://www.gotquestions.org/disciple-whom-Jesus-loved.html)