Holden's stop at the Grand Central Station and, in particular, his meeting with the two nuns and conversation that he has with them occurs in Chapter Fifteen. I would want to suggest that what is important about this meeting is the way that it challenges Holden's simplistic world view and the division that he has drawn between the innocence and goodness of childhood and the superficiality of evil. Up until this stage, every adult that Holden has any dealings with is "phony." However, the nuns are definitely genuine and not "phony." They are presented as being sympathetic, intelligent and genuinely interested in Holden, talking about literature and not getting annoyed when Holden blows smoke in their face. The way that Holden appreciates his time with them is obvious as well:
I said I'd enjoyed talking to them a lot, too. I meant it, too.
In addition, not only do they challenge Holden's ideas about adulthood, but they also act in a way that does not fit in to Holden's pre-conceived notions about religion. This all suggests the larger theme of the novel, which is Holden's own coming of age, and his realisation that his view of the world is not accurate and needs revising.