I think that when Nick tells the reader that “Everyone suspects himself of at least on of the cardinal virtues, and this is mine: I am one of the few honest people I have ever known.” it shows that not only that he is a credible source for the events which he will talk about later on in the novel, but also that almost everyone around him is dishonest.  This statement has meaning out of context as well, its commenting on how dishonest EVERYBODY is, so few people always tell the truth because sometimes it is just easier not to, or because being honest can cause dramatic situations.  I thought it was interesting how he suggested that everyone suspects themselves of at least one cardinal virtue which I completely agree with.  Everybody thinks that they have at least one thing which they think they are particularly good at, or a trait that they have by nature because although most people cannot admit success, even fewer people can admit complete failure.