Themis, I have read the Bible from cover to cover and gone over it and studied it for more than 50 years as well as reading many commentaries, etc., etc., etc. You won't find very many Christian theologians who don't believe that the Gospel of John was written mostly by John.
List the 'very many' theologians who believe the Gospel of John was written mostly by John.
The gospel identifies its author as "the disciple whom Jesus loved." The text does not actually name this disciple, but by the beginning of the 2nd century a tradition began to form which identified him with John the Apostle, one of the Twelve (Jesus's innermost circle). Today the majority of scholars do not believe that John or any other eyewitness wrote it, and trace it instead to a "Johannine community" which traced its traditions to John; the gospel itself shows signs of having been composed in three "layers", reaching its final form about 90-100 AD. According to the Church Fathers, the Bishops of Asia Minor requested John, in his old age, to write a gospel in response to Cerinthus, the Ebionites and other Hebrew groups which they deemed heretical. This understanding remained in place until the end of the 18th century.
The Gospel of John developed over a period of time in various stages, summarized by Raymond E. Brown as follows:
An initial version based on personal experience of Jesus;
A structured literary creation by the evangelist which draws upon additional sources;
The final harmony that presently exists in the New Testament canon, around 85-90 AD.
In view of this complex and multi-layered history it is meaningless to speak of a single "author" of John, but the title perhaps belongs best to the evangelist who came at the end of this process. The final composition's comparatively late date, and its insistence upon Jesus as a divine being walking the earth in human form, renders it highly problematical to scholars who attempt to evaluate Jesus' life in terms of literal historical truth.