“To my disappointment I now realized that to know all is not to forgive all. It is to despise everybody.”
― Quentin Crisp, The Naked Civil Servant
On Grammar (EL&U content)
Grammatically speaking, the key to understanding the actual meaning of this quotation is recognizing its parallel construction and the consequent equating of impersonal pronouns, or at least substantives if you prefer to call them that. Because of the parallelism of construction, the two uses of all as direct objects of their respective verbs are intended to co-refer to what the third direct object at the end refers to: everybody.
That’s as far as we can go to analyse the quotation as a matter of the language’s grammar and form. Anything beyond that is best understood only by knowing the broader context of his writing there. Here’s one possibility, but these things are better answered at our sister-site for Literature.
On Meaning (not EL&U content)
Crisp is saying that once you completely know people in all their private thoughts and histories and habits and peculiarities, the things that make each of us unique, rather than this intimate knowledge being a reason to find them all eminently human and therefore worthy of forgiveness, you instead find them worthy of contemptuous disregard. It is an application or elaboration of the familiar English-language proverb that runs:
Familiarity breeds contempt.
Which Wiktionary explains means that
The more acquainted one becomes with a person, the more one knows about his or her shortcomings and, hence, the easier it is to dislike that person.
Do not mistake that proverb or Crisp’s own statement as some universally recognized truth. Rather, they are only the speaker’s own perspective, one that has perhaps been shared by many others but which is not “true” in some logical sense like saying that east is the opposite of west. They may indeed say more about the speaker’s own point of view than about humanity in general.
Indeed a conclusion opposite to Crisp’s was reached by Henry Wadsworth Longfellow when he wrote:
“If we could read the secret history of our enemies we should find in each man’s life sorrow and suffering enough to disarm all hostility.”
― Henry Wadsworth Longfellow