The Gospel does not give a name
Unlike Paul's letters, the Gospel of John does not open by naming its writer. Near its close, though, it points to “the disciple whom Jesus loved” as the one whose testimony stands behind the book. The writer also says that this testimony was written down and is trustworthy (John 21:20-24).
That disciple appears at several key moments. He is close to Jesus at the Last Supper, remains near the cross, and reaches Jesus' empty tomb with Peter (John 13:23, John 19:26-27, John 20:2-8). The book does not plainly say, “This disciple is John,” so it is wise not to claim more than the text itself says.
This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.
Why John the apostle is the traditional answer
From the earliest centuries of Christian history, the usual understanding has been that John, the son of Zebedee, wrote this Gospel. John was a fisherman from Galilee, the brother of James, and one of the first disciples Jesus called (Matthew 4:21-22). He belonged to Jesus' inner circle with Peter and James.
The New Testament also shows John serving alongside Peter after Jesus' resurrection. When they spoke openly about Christ in Jerusalem, the religious leaders recognised that these men had been with Jesus (Acts 4:13). That close, firsthand connection helps explain why Christians have linked John with the Gospel's personal and detailed witness.
A question Christians discuss
Not every reader reaches the same conclusion about authorship. Some scholars think John himself wrote the book. Others think his teaching and memories were preserved by followers in a church community, perhaps with a final editor shaping the finished Gospel. Scripture does not settle every detail of that process.
Still, the book's central claim is not uncertain: its witness comes from someone closely connected to Jesus, and it was written so readers might trust him. John says his selected signs point to Jesus as the Christ, the Son of God, and that believing in him brings life (John 20:30-31).
But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
Why the author matters, and why Jesus matters more
Knowing that the Gospel is traditionally connected with John can help you read it well. John writes with the perspective of a disciple who wants people to see who Jesus is, from the opening declaration that the Word was with God and was God (John 1:1-3) to the risen Jesus meeting his followers.
But the Gospel does not ask you to stop at curiosity about its writer. It invites you to look at Jesus himself. As you read, notice what Jesus says and does, then ask why John chose that moment. Start with John 1, or read one chapter at a time. The book was written to lead people towards faith, not merely to win an authorship debate.