Quote 19
I'd learned this from my aforementioned third best friend, Peter Van Houten, the reclusive author of An Imperial Affliction, the book that was as close a thing as I had to a Bible. (1.65)
Can we consider An Imperial Affection to be a kind of religion for Hazel? Does she think of it like that?
Quote 20
Maybe some people need to believe in a proper and omnipotent God to pray, but I don't. (12.174)
Even though Hazel's not at all religious (in the organized religion sense), she still feels the need to pray when things are dire. She loves Augustus and is willing to go against her intellectual beliefs to try for a miracle.
Quote 21
[…] his dad put an arm around me and kissed the side of my head and whispered, "I thank God for you every day, kid." (19.39)
Augustus's parents really do see Hazel as a source of solace in their time of need. And even though she doesn't believe in a God, she accepts their faith.