is it reliable historically?
According to historians it has some historical value.
Gospel of John
. By the same token, scholars usually agree that John is not entirely without historical value: 
how many chapters does the gospel of john have 21. what is its structure? It has four sections: A prologue, the "Book of Signs", the "book of glory", and a conclusion. The epilogue might not have been a part of the original text.
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Gospel of John [SEP] Historical reliability
The teachings of Jesus found in the synoptic gospels are very different from those recorded in John, and since the 19th century scholars have almost unanimously accepted that these Johannine discourses are less likely than the synoptic parables to be historical, and were likely written for theological purposes. By the same token, scholars usually agree that John is not entirely without historical value: certain sayings in John are as old or older than their synoptic counterparts, his representation of the topography around Jerusalem is often superior to that of the synoptics, his testimony that Jesus was executed before, rather than on, Passover, might well be more accurate, and his presentation of Jesus in the garden and the prior meeting held by the Jewish authorities are possibly more historically plausible than their synoptic parallels.