Character Analysis
Monsignor Darcy is without doubt Amory's mentor. The man not only looks wise, he is wise, which is a shocker for young Amory, who isn't used to people putting their money where their mouths are.
We can tell when we first meet him that he's going to have a good influence on Amory:
When he came into a room clad in his full purple regalia from thatch to toe, [Monsignor] resembled a Turner sunset, and attracted both admiration and attention. (1.1.193)
This kind of admirable dude is a welcome change to the superficial entitlement of Amory's mother, Beatrice. That being said, it takes way more than hanging out with a religious guy to make Amory Blaine into a modest person.
As the narration continues to describe Monsignor, it focuses on his open-mindedness and youthfulness as reasons why young people are so willing to trust him. As the text says, "Children adored him because he was like a child; youth reveled in his company because he was still a youth, and couldn't be shocked" (1.1.194). These awesome qualities lead Amory to be super-trusting of the man.
Also, Darcy is willing to listen to Amory's young philosophical ideas with a level of interest that no other adult has ever shown Amory. And the easiest way into Amory's heart is to make him feel important.
Monsignor Darcy cares so deeply for Amory that he often wishes that Amory were his son. As he says at one point,
"I've enjoyed imagining you were my son, that perhaps when I was young I went into a state of coma and begat you, and when I came to, had no recollection of it… it's the paternal instinct, Amory." (1.5)
Even though Monsignor Darcy has sworn a vow of celibacy (no sex) for life as a Catholic priest, there's a big part of him that has always wanted a son. And Darcy thinks of Amory less as a spoiled-rotten kid and more as his own bouncing baby boy.
Hey, at least one adult is looking after Amory in his formative years, right?