How did Marcie know that? There were only certain ways for a person to know that the gods were watching out for them... But, of course, Marcie's 'know' didn't have to be literal. Sometimes you 'knew' things because you felt them. Marcie could feel as though Hecate and Athena and Aphrodite were watching out for her in a warm fuzzy way, and not because she'd run into them on a street corner and talked.
(Hecate especially. He couldn't imagine casually running into Hecate on a street corner. Hecate was more of an intense, dark room, smells-like-ylang-ylang goddess, in Cathal's opinion.)
"It sounds nice," Cathal told her with a smile, not wanting Marcie to feel like she couldn't say any of this, even as he put his own weird feelings aside. "There's nothing wrong with having faith in something bigger than yourself."
It felt hypocritical to say when Cathal had abandoned faith in all of that.