When reading the Fourth Gospel, some passages stand-out as wrong.
How these passages got-into the text I am reading is not really important to me - clearly there are many times and ways it could have happened; and equally clearly, when dealing with divinely inspired and sustained texts the normal understandings of secular 'historical' scholarship are inadequate and misleading.
(Mostly, the provenance of error is unknowable because there are an open-ended number of possibilities; it is the provenance of truth which is vital.)
The Fourth Gospel has a form, a method, a shape - overall it is a highly-perfect work, perhaps the most perfect of all sustained works; this means that errors stand out. Furthermore, the gospel is true, and known-as-such; so wrongness stands-out.
From Chapter 20:19 Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
20 And when he had so said, he shewed unto them his hands and his side. Then were the disciples glad, when they saw the Lord.
21 Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you.
22 And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost:
23 Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.
24 But Thomas, one of the twelve, called Didymus, was not with them when Jesus came.
25 The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe...
The italicised are wrong, furthermore they are detached from the narrative - which runs straight from verse 20 to verse 24. .
Verse 21: The analogy between the Father sending Jesus, and Jesus sending the disciples, is basically-false.
Verse 22: Jesus has previously explained at length that the Holy Ghost cannot come to the disciples until he has ascended to Heaven - so he cannot breathe the Holy Ghost onto them at this point.
Verse 23: The bald statement that the disciples are being given power to remit/ retain sins (whatever that may mean) is at odds with the rest of the Gospel. The possibility that the disciples be given power over sins, or that such a remission is even necessary or coherent, is in stark contradiction to the overall teaching of this gospel - in the sense of having nothing-to-do-with the rest of the gospel. Furthermore (in this gospel), whenever Jesus says anything as important as this would have been; he always says it several times, in several ways, generally in several places; to ensure it is appreciated and understood.
(Why were these verses wrongly interpolated? Well, at the cost of contradicting the core gospel message; it seems fairly obvious that these verses imply that Jesus ordained his disciples as a priesthood analogous in power and status to himself, and as necessary to salvation. Maybe that explains why they were inserted at some point?...)
The Fourth Gospel is - on the one hand - hard to understand; being expressed in an unfamiliar way; on the other hand it is understandable by anyone who gives it sufficient of the right kind of attention - because it is a window onto universal consciousness.
The fact that the Fourth Gospel is a human product, as well as divine, will not block that possibility - because God is on both sides of the situation: as-it-were present in the text and also as a part of our-selves: present (not in perceptions, not in mental concepts) in the thinking of the real self.
Furthermore; if one is reading the gospel for the best kind of reason - that is, as a kind of meditation/ prayer, for personal and direct knowledge (rather than in order to extract from it rules and regulations for general, public communication and control) -- then the process of understanding, or knowing, is itself of great value and greatly satisfying.
Understanding the Fourth Gospel is not really a finite task that could be done and finished-with; nor is it 'objectively' checkable whether or not the task has been achieved. This is because when talking about the Fourth Gospel - we are only talking about it.
Knowledge comes first - but the communication of knowledge, and its reception, is a different matter altogether.