What event caused Gifty to lose her religious faith in the novel, Transcendent Kingdom?

Gifty's brother, Nana, died from a heroin overdose when she was eleven-years-old, almost immediately after leaving a 30-day stint in a rehab facility. The loss of her brother caused Gifty to lose her religious faith and her ability to trust other people, issues she continues to struggle with over a decade later in the timeline in which she is 28.