Imagine a copy of the Gospel of John was dropped from an airplane into a tribe of remote people who had no other books of the Bible. Imagine that prior to the Gospel drop, they had never heard of Jesus, Christianity, or any other aspect associated with the Bible. Also imagine that the title was removed. It is impossible that they would identify the author as the disciple named "John." What reason would they have to do so when the only two “Johns” mentioned in the Gospel are John the Baptist who was killed and Peter’s father. They would have no reason what-so-ever to think Peter’s father wrote the account.
Now imagine the Gospel was dropped with the heading “According to John.” Would they draw the conclusion that John is the name of the beloved disciple? This is far more likely considering John 21:20-24. Verse 20 clearly identifies a man nearby as the disciple Jesus loved, who was reclining at the table in John 13. Peter asks Jesus about this nearby man and a brief discussion about the man transpires. Then verse 24 says, “This is the disciple who is bearing witness about these things, and who has written things, and we know that his testimony is true” (ESV). Given the title of the work, “According to John,” the tribe would likely only come to two possible conclusions. First, they would likely conclude that the beloved disciple wrote most if not all of the Gospel. Second, they would conclude that either, 1. John was the name of the beloved disciple, 2. John was the name of the editor that redacted the beloved disciple’s work and added the final few lines as a tribute; or 3. that John was the name of the beloved disciple who took notes which an editor or group of editors redacted, added a few lines at the end and named after him.
These two exercises demonstrate the significant evidence found in the of the title of the work. That being said, it is uncertain that this was the original title. However, Burge argues that the earliest Greek manuscripts known to us today do bare this title (1992, 40). In addition, these two exercises isolate the huge body of external evidence, which includes other works contained in the Bible as well as the writings of other people in the first and second centuries. Given this body of evidence, it is most likely that John was the name of the beloved disciple and did write all or nearly all the gospel that bares his name.
In light of the evidence, if the beloved disciple is John—as the external evidence demonstrates—but John is not the author of the Gospel of John as some scholars claim, a lie is present in the Book of John, seriously bringing question upon the credibility of the Gospel. However, there is substantial evidence (which is not discussed here) that supports that John is both the beloved disciple and the author of the Gospel of John.
Therefore, I have to conclude that John, the son of Zebedee, who was also the beloved disciple, is the same John who wrote the Fourth Gospel.
Burge, Gary M. Interpreting the Fourth Gospel. Grand Rapids, Mich: Baker Book House, 1992.