When the Jesus Seminar assessed the Gospels they decided that none of the sayings which are attributed to Jesus in the Gospel of John were definitely uttered by him.
The Gospel of John is a highly developed piece of theology known as Christology. And the sayings attributed to Jesus are theological statements rather than the teachings and parables which appear in the Synoptic Gospels (Matthew, Mark, and Luke).
So rather than a portrait of the historical Jesus, the Gospel of John describes ideas which grew up around the figure of Jesus after his death.
That is not to say that it has no value.
But it must be remembered that the Gospel of John was written 100 years after the birth of Jesus. It differs dramatically from the other Gospels in many ways and may in fact be a less reliable guide to the historical Jesus.