Nick says that reserving judgments is a "matter of infinite hope." This is what he means by that:
Reserving judgment means that Nick delays his final conclusion about someone's character. In other words, even if someone behaves badly, Nick tries to not judge them, to give them a second chance.
By doing so, he creates infinite hope, which means he is constantly hoping that the person will redeem themselves, that they are actually good and only had a momentary ill decision.
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