Nick Carraway says, "I am one of the few honest people that I have ever known." Is this true?
Nick Carraway is not an honest person. We will get back to the rest of the interesting passage in which Nick declares his honesty in a moment, but for now, let's start with the big picture: Nick aids and abets Gatsby and Daisy in having an affair. We may sympathize with him--Tom is adulterous and brutal--but, nevertheless, it is not precisely honest to help a married woman, especially one whose husband is your friend, have an affair with another man. Nick carefully keeps this concealed from Tom, as we realize when Tom recognizes, with surprise, that everybody but him knew this was going on.
Getting back to the passage about honesty, Nick first ponders Jordan and her possible cheating at golf, concluding that she is "incurably dishonest." Then he feels, in a flash, that he is falling in love with her, after dating her casually: has that careless dating, in itself, been deceptive? Has he encouraged Jordan to believe he feels more than "tender curiosity" about her? Whether yes or no, he then admits to the reader, if not to himself, his dishonesty in his relationship with a woman back home. He calls it a "tangle" and says that he has been signing his letters to her "Love, Nick." However, she seems to repulse him. He says "all I could think of was ... a faint mustache of perspiration ... on her upper lip." He says "there was a vague understanding" that has to be "tactfully broken off" for him to be "free."
In other words, he has been dating Jordan while stringing another woman along. That may be human behavior, but it is not precisely honest behavior. It is at this point, however, that he declares "I am one of the few honest people I have ever known."