If Nick were to forget this idea, that not everyone was born with (or taught by their parents/teachers) the same sense of honesty, then Nick fears he might become judgmental. Nick prides himself on being honest and objective. Nick recalls his father's advice which has informed this perspective:
“Whenever you feel like criticizing any one,” he told me, “just remember that all the people in this world haven’t had the advantages that you’ve had.”
Nick recognizes that he's been fortunate to have been raised to be an honest person, one who is "inclined to all reserve judgements" of others. If he loses this attitude, he would become less tolerant and more inclined to pass judgments on people without considering their backgrounds and personal histories. For example, on the surface, Gatsby seems showy and pretentious. Nick reserves such judgments and eventually discovers that he admires Gatsby's hopeless romanticism.